Silencers, muzzle blast and accuracy. Part 2

[David Snook]

Here follows some observations and thoughts about the relation between silencers, muzzle blast and accuracy - and more – Part 2. This follows my earlier post on this topic where I received much valuable feedback.

During my investigations it has become painfully obvious that my quest has opened up more questions that answers, however I think that I have managed to distil some interesting information that might be useful to like-minded shooters.

One thing that I have learnt, those seeking extreme accuracy can be driven mad by various obstacles put in their way by nature, physics and good old Murphy’s Law. One day you can have perfect groupings the next – rubbish. I will do my best to shed a little illumination on these frustrations.

Meanwhile my various notes, theories and observations are up for comment and discussion where any constructive criticism will be carefully considered.

1. Muzzle blast causes inaccuracy: This is the fundamental premise of this post. There were some doubters at first, but now there seems to be general agreement that this effect not only exists, but also is quite commonplace.

2. Blast removal is not the best answer: Prevention is always better than cure and the actual design of the valve, transfer ports, barrel length and hammer can effect blast greatly. As a rule of thumb, noisier, inefficient guns (obviously without a silencer fitted) are more prone to blast related problems than guns that are naturally quieter and efficient by design (again without a silencer fitted). Valve oscillation can also contribute to inaccuracy, hammer/valve bounce not only wastes air but also contribute to buffeting of the pellet as it leaves the barrel. Quite a few shooters have noticed that groups have tightened up after their gun has been modified to produce less blast. This report comes from Darren Petts who has also experimented with air strippers, see below:

Gun is shooting fine. There's a noticeable reduction in air blast. The pellets are actually landing a couple of pellets higher at 55 yards on slightly less power. I can only assume that the smaller air pulse is deforming the pellet less and giving me a slightly better BC. The groups have tightened considerably at distance too. I'm making a set of air strippers to fit inside the std pre-silencer which should further aid in pellet stabilisation.

3. Any device fitted to the front of a barrel is potential trouble: As far as accuracy goes, if anything can go possibly wrong – then it will! The simplest way of eliminating problems with muzzle brakes and silencers is not to fit them in the first place. This is not so easy where some guns have built-in moderators or bull barrels and in some applications such as hunting, a moderator is a must!

4. Muzzle brakes are not always what they seem: The traditional muzzle brake works by directing air upwards or backwards or both. My testing has shown that brakes that direct air in one plane can deflect the pellet in the same direction (i.e. deflecting the blast upwards will skew the pellet upwards as a rule) and at the same time impart destabilising forces on the pellet. There are notable exceptions to this rule, which will be covered later. The best kind of muzzle brakes usually has some kind of internal air stripper and vents the air in a radial direction (i.e. not in a particular direction) around it. A badly designed muzzle brake can actually contribute to inaccuracy.

5. Air strippers: While air strippers might not be the cure-all that they might seem a well designed one does seem to improve accuracy, and would probably benefit a gun with a large blast signature more that a more efficient, quieter gun with a smaller blast signature. Some highly tuned “race” guns have got some very effective air strippers that must contribute to their accuracy. Here follows an abridged account of some experimentation from Darren Petts, please note that this air stripper vents the blast upwards, which I do not recommend:
I made my air stripper using 4 forcing cones fitted into a standard pre-silencer with 6mm holes to vent the air vertically. Initial trials were promising, bar the odd 1 in 15 or so flyer that went up to 2 inches astray in any direction. I opened the pellet pathway from 5mm to 5.5 mm and this cured the problem. The pellet didn't appear to be hitting anything, although I assume it was just so close at 5mm that the boundary layer of air was disturbing the pellet and caused destabilisation. A 5.5mm hole cured this (the drill was 5.5mm so the
hole will be a small fraction larger). The groups are now typically 0.5" at 55 yards with the worst of ten 5-shot bench rested groups today being 0.7". This is a fair improvement from what I was getting earlier with groups up to 1.0" which was again an improvement on the 1.25" I was getting before lean-charge.

5. Silencers: Silencers can cause allsorts of problems, but correctly designed and fitted they can increase accuracy by adding mass to the end of the gun, making a more stable aiming platform and they can also strip air away from the pellet. One of the biggest problems is concentricity, if a pellet passes too close to a surface when just leaving the barrel, a boundary effect of high pressure blast between the surface and the pellet cab be caused. This causes deflection resulting in a POI shift which I call skew. Apart from skew, destabilising forces are also imparted to the pellet - causing wobble with attendant reduction in accuracy. A well known silencer was causing accuracy problems on my experimental Mk3 fitted with a heavy 19 inch honed Walther threaded barrel, there is no built-in moderator fitted. This gun is capable of pellet sized 5 shot groups at 35 yards in still conditions. The groups were opened up to over 20mm when the silencer was fitted, after removing all the innards from the silencer; the groups were still about 20mm. When I inspected the silencer I noticed that the 11.5mm lead-in to the silencer chamber had a 6mm restrictor hole 18.5mm from the barrel crown, before the silencer chamber opened up. The blast had no expansion chamber to expand in and was obviously being reflected by this restriction and interfering with the pellet as it left the barrel. I drilled out the restriction with a 11.5mm drill and the group size was reduced to less than 10mm in breezy conditions at 35 yards.

6. Power and accuracy: Reducing power can sometimes increase the accuracy of a gun. A colleague noted a strange phenomenon when adjusting his gun to just under 11.9FPE on a regulated gun. The velocity began to vary and groups started to open up. When reduced to the “magic” 777FPS the gun sounder a little quieter, the groups tightened and there was a slight POI change UPWARDS! After much deliberation we decided that the following effects re responsible:

Most modern sub 12FPE guns are designed to limit power to a maximum of just over 12FPE, no matter how you adjust them. This is done with hammer weights, porting and valve sizes. Turning the power up to the maximum legal limit can make it start to operate in its deliberately inefficient design area and will do nothing except giving it a slight increase in power and make it inefficient, noisy and increase blast.

The blast was opening up the groups, and reducing the power in turn reduced blast and reduced group size, but why did the gun appear to shoot flatter? We believe it is to do with pellet flight BC (not pellet BC). This is where the BC of a wobbling pellet is apparently worse than a pellet flying true. Paradoxically slightly reducing the power of a gun can actually give you a flatter trajectory with a higher velocity at the target. Earlier Darren associated this effect with pellet deformity; we now believe that the pellet wobble explanation is more accurate.

This effect can also happen when shooting an unregulated gun. All unregulated guns have “sweet spots” of some description. Not only do sweet spots generally produce the highest power, but also indicate the gun’s most efficient operational pressure, reducing blast and in so doing, gives the greatest accuracy.
To be continued……….

[David Snook]

Continued

7. Tell tale indications of inaccuracy: When fitting a brake or a silencer and the POI shifts noticeable, then as a rule you can expect trouble, especially if it is right to left. If the device is heavy then the barrel will droop slightly shifting the POI down, this does not necessarily point towards accuracy problems in this case. If your gun is grouping acceptably at 25 yards and then opens up at 45 yards or more, you might well have blast related pellet instability problems.

8. Measuring the accuracy of your gun: I suggest 10 shot groups. They should be shot in indoor or still conditions. If there is a wind blowing up or down range it will raise or lower the POI. If it is blowing left to right or vice versa the POI with go left or right and up and down slightly. This is because of spin related lift effect similar to golf ball flight. Do not confuse this effect with inaccuracy problems.

9. Barrel deflection: When investigating accuracy I found that a non-blast related problem had an effect on accuracy. If the barrel was twisted slightly to one side by a dislodged figure of eight barrel restrictor, then there was a POI shift and degradation of accuracy, even without any silencer or brake fitted. I think it is something to do with the barrel being held in a slightly bowed position under tension by a sideways force.

10. Some silencer and muzzle brake design pointers: It is crucial that the device allows blast expansion as close to the muzzle crown as possible and this are is left clear of restrictions that can cause reflections or focus blast. I suggest that the first element of any internal disc components is at least 3 pellet diameters away from the muzzle crown (maybe someone can give some practical examples of suitable spacing) with enough volume to allow expansion between.

I hope that this has been of some interest and maybe has helped a few shooters experiencing mysterious accuracy problems. If anyone has any more information I would be glad to hear it, even if it contradicts my findings!

David

[ingoongapa]

hi i havent read all of this yet ut it might have some other interesting things........




AIRGUN BALLISTICS
Some preliminary remarks.
By Robert Beeman
17 August 2003


When adult airguns first started to became widely popular in the United States in the 1970s, the power of airguns generally was indicated by their muzzle velocities, expressed in feet per second (fps) or meters per second (mps). Although Crosman started their air rifle production in 1923 with .22” caliber, it was .177” caliber that became America’s favorite caliber until late into the 1980's. At that time, .177” caliber accounted for over 95% of the sales of adult airguns in the U.S. and there was a rather narrow range of pellet weights and styles available. Because of this, and because velocity generally is more understood and recognized by the public, velocity figures were used to give a feeling for the power of the various airguns available. At that time, it was like we were comparing the speeds of different kinds of cars, not locomotives and bicycles. When we were considering sporting air rifles of the same caliber, all firing rather similar projectiles with rather low energies, “effective accuracy” was the key consideration.

Velocity indeed is a key factor in airguns, perhaps even more so than in firearms. The higher the velocity is, the flatter is the trajectory of the pellet. Thus higher velocity means that we can be less concerned with the exact determination of range, as a faster pellet would be closer to the line of sight than a slower one. This is one of the key reasons that hunters of small game often select high velocity cartridges. It was realized that most shooters cannot accurately judge distance in the field, few of us can. With small targets, it is often more important to have a flat shooting gun, with which the shooter can more easily hit the small critical area of that target, than to have a projectile with greater energy. Higher velocity means a faster "lock time", or exit time, the amount of time that elapses between the actual trigger release and the projectile emerging from the muzzle. A faster exit time means that the flight line of the projectile will be closer to the shooter's intended sight line. The sight line is, of course always moving around as the shooter is aiming. Shorter lock time is of even greater importance to the new and poor shot than it is to a very highly trained shooter who can control the gun's movement to a greater degree after trigger release. Higher velocity means greater effective accuracy; this is of very special importance when one considers the extremely small critical areas of the kind of game and other targets fired upon by airgunners. We must be very careful not to confuse "effective accuracy", the only kind of accuracy that really counts in the field, with "theoretical or benchrest or machine rest" accuracy.

Higher velocity also can mean greater actual accuracy. While working on an airgun injury case, the Crosman Airgun Company and I conducted independent tests which revealed that a .177” (4.5 mm) caliber match air rifle became more and more accurate as its muzzle velocity was increased from 400 fps to 700 fps. The very top match air rifles fire a 177” (4.5 mm) pellet of about 8 grains at muzzle velocities of about 550-650 fps. It had long been felt that muzzle velocities in the 550 to 650 fps range resulted in the greatest air rifle accuracy. This conclusion was reached because most match airguns fired in this velocity range and they were the most accurate airguns in existence. While this may be the optimum velocity for some airguns, the selection of these velocities for indoor match shooting also may be a function of other factors such as custom and the ease of manufacturing, cocking, and shooting a gun of that velocity.

Inherent accuracy may or may not increase, as velocity is further increased over this indoor optimum, but, for airguns used outdoors, “effective accuracy” greatly increases. This is because, as velocity is increased, not only does the trajectory becomes flatter but side winds do not have as much time to affect the pellet’s path. Thus a higher velocity airgun makes it easier to place the pellet more precisely on the intended targets at varied, and generally unknown, distances. The drawbacks of lower velocity are not factors to paper target match shooters. They fire in windless, indoor ranges at exactly known distances.

The increase in actual effective accuracy, which can accompany higher velocity in outdoor airguns, perhaps is the primary reason why so many American gun makers have increased the velocity of airguns that are used outdoors.

To get some perspective on airgun velocities, consider the muzzle velocities of some well-known guns: A typical "BB" gun imparts about 250 to 350 fps to a light (about 5 grains [0.32 grams], .174” (4.4 mm) steel ball. A .22” ( 5.5 mm) rimfire cartridge rifle has a regular-speed muzzle velocity of about 1025 to 1145 fps. Ten pumps in a Daisy Powerline 880 or Crosman Powermaster 760 BB/pellet pneumatic will fire pellets at about 570 to 670 fps. Ten pumps in a Benjamin M342 .22 caliber air rifle, produces about 640 fps. Ten pumps gives about 605 fps in the .177” Crosman 1400 or 695 fps in the .20” Sheridan air rifle. The muzzle velocity of a Beeman R-1 air rifle ranges about 590 to over 1100 fps, depending on model and caliber. A .38" Special (9 mm) firearm or .45" (11.4 mm) ACP firearm wadcutter bullet moves at about 770 fps muzzle velocity, but is extremely dangerous due to its great weight. In terms of the more familiar miles per hour, the BB gun sends out its projectile at about 170 miles per hour, while a top level adult air rifle will rush its projectile out at over 750 miles per hour. Plaintiff lawyers in airgun cases often dwell on the velocity of airguns as a measure of their danger. However, one must temper any considerations of velocity with the mass of the moving object; obviously most of us would choose being hit with a BB at 170 miles per hour rather than by an automobile, or even a hard baseball, going “only” 60 miles per hour!

DEFINING AIRGUN POWER:

Although the muzzle velocity of airguns has been the primary yardstick by which adult airguns have been compared in the past, this figure does not have a lot of significance in the real world. To start with, it is velocity at the target, not at the muzzle, that really counts in field use of an airgun. The field shooter and hunter is most concerned with how hard his projectile hits the target, and that involves not only velocity but projectile weight, projectile shape, and numerous environmental factors. An airgun projectile ricocheting from a hard surface, or even touching a leaf or grass blade, may lose much of its power and accuracy.

Some individuals, notably those who feel more comfortable when the world is reduced to fewer and simpler considerations, like to say that muzzle energy is the only true measure of an airgun's output. While that might be considered true from a physicist's standpoint, because muzzle energy is a combined function of both projectile velocity and mass, it downplays some extremely important considerations, such as trajectory, wind deflection, penetration, expansion, wound channel size, and the very inertia of the projectile.

Nevertheless, muzzle energy is the most meaningful way to compare the power of airguns. Giving gun power in terms of energy is the most practical way to compare guns of different caliber and projectiles of considerably different weight. This is now necessary as the market has become more sophisticated and it is important in preventing unfair, unrealistic legal comparisons of airguns and firearms. Comparing guns by energy, generally muzzle energy, also better compares the true efficiency of various guns. A quick examination of the Beeman, and other top line airgun catalogs, shows that the more powerful spring piston air rifles are much more efficient in .25” caliber than in smaller bores. Some of the most powerful spring piston airguns, such as the Beeman Kodiak, generally are not even offered in .177” caliber because the powerful air flow of such guns literally is strangled by the small bore.

[ingoongapa]

continued.......

Airgun energy generally is expressed in foot pounds. It is well worth considering, in practical terms, what a foot pound represents. One foot pound is the energy that a one pound object releases when falling one foot (ignoring the friction of air, which, from a very rough practical sense over very short distances, we may do for very dense objects). Thus, one can roughly consider that a gun firing with a muzzle energy of 12 ft./lbs. is about the same as dropping a 16 oz. hammer head from about 12 feet. Extend that to a 30 ft. drop for a 30 ft./lb. gun. Imagine being hit by that falling hammer head and you are doing a very crude visualization of the gun’s potential hitting strength.

II. Penetration:

In European airgun factories, airguns commonly are tested by firing against a shielded hard steel "splash plate". If the pellet explodes into fragments the gun is considered to be in good condition. Note that a magnum spring-piston air rifle can continue to explode its pellets against a steel plate to over 35 yards!

Americans frequently test their airguns by firing into soft wood. Unfortunately wood probably is one of the worst possible testing materials because its grain, type, and condition varies tremendously with wood species, dryness, etc.. However, some very rough "ballpark" ideas of penetration can be had by this method. These notes refer to .177” caliber airguns. Guns firing at 630 fps will usually completely bury their pellet into soft pine or redwood. An 800 fps sporter will frequently tear completely through a 1” finished board, splintering out the rear as the pellet leaves! Aluminum beverage cans provide more uniform testing material, but maximum penetration depends on hitting them exactly square. A 640 fps match test air rifle could go right through six cans. A magnum sporter could rip through over 10 cans. Even a match air pistol could go completely through four!

Ballistic putty is one the best materials for testing airgun projectile penetration. It is relatively uniform and so dense that penetration depths can be rather easily measured from the surface. It is far denser than flesh. A soft lead, round nose pellet from a 780 fps gun will penetrate a total of about 3/4" into this material at room temperature at about one foot firing distance. (Don't forget to add the length of the projectile when measuring penetration depth!).

There are several other good materials that can be used for penetration testing. The outstanding airgun author Tom Holzel champions the use of Ivory soap bars. Most forensic and ballistic laboratories use various specialized media such as ordnance gelatin and ballistic clay. We will consider those materials in separate articles.

Penetration is not completely a function of velocity, of course. A hard, pointed pellet like the Sheridan or Prometheus has excellent penetration but has less shock power than a mushrooming soft lead pellet. A sharply pointed pellet, like the Silver Jet pellet, also penetrates deeply but its softness allows some expansion for shock value. The Crow Magnums hollow point maximizes expansion. A really hard projectile, such as a steel BB or dart, can have quite great penetration power, but its capacity for injury is greatly reduced by the low amount of tissue damage.

Just as with high velocity firearms, over-penetration can be a problem with airguns. Some airgun projectiles may make a great impression by the number of telephone book pages they can penetrate, but the wound channel such pellets produce in the field may be so tiny as to have almost no knock-down effect. Unfortunately, the "acupuncture" effects of such projectiles and others, such as steel core pellets or darts, may mean more than just the loss of game to the shooter; they may mean a long, cruelly lingering death to an injured animal perhaps without the shooter even knowing that he scored a hit. Even pointed lead pellets may have undesirable over-penetration if used on very light game at close range. Such prey calls for the use of a hollow point pellet, or at least a flat head wadcutter pellet.

PELLET FLIGHT

The typical airgun projectile, with its characteristic diabolo (not diablo, that’s Spanish for devil!) hour-glass shape, is basically different from that of most firearm projectiles and thus some information about bullet performance may not apply nearly so well, or at all, to pellet performance. At the present time very little empirical information on pellet performance has been published. Much of that which has been published may have very little to do with the most basic points of pellet performance. For instance, many airgun shooters become very concerned if one batch of pellets varies in weight from another. Such differences may simply require a slight change in sight adjustments. Of somewhat more importance is the matter of weight variation within a given batch of pellets, but the significance of even this absolutely pales in comparison to the importance of the evenness of weight distribution within each individual pellet. Pellets with quite a significant weight variation could be extremely accurate in test firing, especially at close ranges, before trajectory differences become pronounced, if the mass of each pellet is evenly distributed within the design of that pellet. However, perfection of weight distribution, especially within a series of randomly selected pellets, is virtually impossible, but such uniformity is one of the keynotes of pellet quality - and thus cost and performance. As in so many matters, there is no free lunch in the pellet business.

[ingoongapa]

continued.....



The displacement of weight from a perfect arrangement within the design of a given pellet has both radial and longitudinal components. Considering only the radial component, a perfectly balanced pellet would have the center of weight distribution at the axis of the pellet. Pellets in the real world virtually always have their weight center slightly off center from the axis. As the pellet moves up the rifle barrel and a spin is imparted to the projectile by the rifling, the centerpoint of weight is going to follow a helical path rather than the straight axial path that a pellet itself makes as it passes up a perfectly straight bore. In pellets of even only moderate quality, the diameter of this helical path certainly must be less than a thousandth of an inch. The pitch of the helix is going to be very long, matching the pitch of the rifling. Thus, if this helix should somehow magically become visible, it would certainly appear, except under extreme magnification, as a perfectly straight line.

Considering the above should not be difficult, but it requires a considerably greater stretch of the imagination to understand what must happen at the muzzle. A weight on a string swung around above ones head will follow a circular path. Consider that the pellet, with its off axial center of weight, is being spun around and around in the rifled bore but is restrained from flying to the side by the bore of the gun. Remember that because the projectile is swiftly moving forward, the path of the weight center follows not a circle, but an extremely elongated helix. When the spin of the weight center exits the muzzle it is going to cause the weight to fly off at a tangent from the spin, just as the weight on a string flies off at a tangent from the circle when the string is released. However, since the pellet's weight center has been traveling in a helix rather than a circle, it will fly off at a tangent from this helix. Depending on where it leaves the helix, this tangent might parallel the bore axis or point at some angle away from it, more likely away. However great or small the deviation of this tangent is from the axis of the pellet and the gun’s bore, two things must be kept in mind. First, since the tangent is flying off from a helix that has such a long pitch and such an extremely small radius, the helix would appear as a straight line if visible to the naked eye and the actual deviation from the axis is going to be extremely small. The deviation from the axial direction will be perhaps less than a millimeter to only a very few millimeters at a distance of ten meters from the muzzle. Second, we must not confuse this tangent of force with the actual path of the pellet. This tangent of force is simply an off center line of force attempting to push the comparatively large mass of the pellet off from the forward, axial path on which its forward inertia is taking it. However, we can see that a pellet with unevenly distributed weight, which is true to some degree of virtually any pellet, is going to start to tip, or "yaw", upon exit from the muzzle and it is going to be pushed further and further away from the axial path upon which inertia alone would take it. The uneven weight distribution thus has caused the pellet to start to tip at the muzzle and this tipping is going to become more and more pronounced, causing an ever increasing spiral of the flight path which will result in greater and greater deviation from the axial line of flight until a point of chaotic instability is reached. The pellet will then begin to tumble and become extremely inaccurate. Of course, the pellet is also subjected to force lines from the continual pull of gravity and from air movements caused by wind and heat waves. The above factors also probably are involved with the accuracy and point of impact changes that occur when a gun is tilted from the position in which it was sighted-in. If there is any damage, even very minor, at the muzzle or if the muzzle is cut even a microscopic amount off from square, the muzzle may impart a good deal of instability to the pellet. The last one millimeter of a barrel can have more effect on accuracy than the entire rest of the barrel!

The "conventional wisdom" is that a small variation is going to have less effect on a large projectile than an equal variation in a smaller projectile. However, in the absence of experimental evidence to the contrary, I believe that an airgun pellet's shape might be a greater factor than its mass under many conditions, within the normal range and velocity of airgun pellets. The length of most larger bore pellets is shorter in relationship to their diameter than in smaller bore pellets. It is possible that this relatively greater length of smaller bore pellets results in them tipping less, having less yaw, due to the tangent of force which develops when unbalanced pellets emerge from the muzzle. Also, the greater relative length of tail on the smaller diameter pellets may give a relatively larger area for air pressure to force the gyrating pellet back onto course. Since the tail of a diabolo pellet has a shape much like the tail on a shuttlecock, a feature not shared with most firearm projectiles, there may be a significant contrast here with typical firearm projectile behavior. Most firearm projectiles simply do not have most of their mass in the forward end with a large, flared, stabilizing tail behind. Of course, a round ball, like a BB, or a lead ball, is not going to have yaw, or any functional result due to tipping, in the usual sense, as a sphere presents the same shape surface regardless of how its axis may tip. A very uniform ball may have quite excellent accuracy to a surprising distance because of this.

[ingoongapa]

continued....



The commonly observed greater accuracy of smaller diameter pellets versus larger diameter pellets may also reflect a difference in manufacturing tolerance in some designs. Almost all the attention to producing highly accurate pellets has been focused on .177” caliber pellets. The tolerances and sorting of larger pellets usually has not been held to as rigid a standard. Also, the design and configuration of a pellet which is highly accurate in .177” caliber may simply not be as conducive to stability, especially without careful consideration of proportion, in larger diameter pellets. The manufacturing perspective and tolerance in making .177" caliber barrels versus larger bore barrels may be a major consideration in some cases. Further, when comparing airguns which are supposedly identical, except for caliber, one generally is comparing guns with the same outside barrel diameter. The smaller bore barrel has greater weight and stiffness and thus considerably less barrel oscillation during firing. This difference is not as pronounced when comparing firearms of larger bore diameters. Also, as is the case in so many considerations of airgun performance, minute differences of various factors, which might have a great effect on airgun performance, might simply be swamped by the relatively tremendous force and projectile mass of firearms. Thus differences which would favor the accuracy of smaller projectiles in airguns may have little or no significance in firearms. We are just starting to understand airgun ballistics, but it is clear that we cannot simply consider them as if they were small firearms. Many aspects of airgun ballistics will surely be shown to be partially or even fundamentally different.

Sometimes, experience reveals that theory springs from experience and not the other way around. After very extensive testing of Beeman and RWS magnum air rifles, the outstanding airgun author, Tom Holzel, found that .25” caliber clearly was the most accurate of the four calibers from .177” to .25”.


Testing and Studying Airgun Ballistics: Airgun ballistics as a field is perhaps some decades behind firearm ballistics. Some would say that we are only now entering the 20th century. However, we have a tremendous advantage that the firearm testers did not have in 1900 or even very late in the century. We have some wonderful instruments and programs to speed the development of our field!

Once you have stepped up from using aluminum cans, Ivory soap, or ballistic putty you will need use of a chronograph. This simply is an instrument that measures the time a projectile takes to travel from one point (start screen) and another (closing screen). No longer do we have to put up with having to actually cut replaceable screens with our projectiles, now we can simply measure the tip between the shadow of a projectile passing one screen and the shadow of it passing the next screen or screens. Over the years, I have used the wonderful Oehler Chronographs, both in our airgunsmithing shop and in the field, and several other chronographs. I still recommend the Oehler machines as the very best, but the Oehler Personal Ballistic Laboratory Model 43, as their basic unit is called, now costs about $800. Check it out at www.oehler-research.com . For travel use and field studies, I have often used the Combro cb-625 Chronograph made in England ( www.combro.co.uk ) which not only is it very inexpensive, but it is so small that you can carry it in your vest pocket! It has two microscopic sensors which read projectile shadows from the sun or even a room light. It just straps onto the muzzle of the airgun to be tested by rubber bands. Although so very handy and so very portable, I often have a hard time getting it to consistently read out velocity figures - getting the pellet to be right on the very tiny, critical sensory path can e very tricky. However, I have used it in many unusual places to be testing firepower - like various law offices when I have been engaged as an expert witness in airgun lawsuits or criminal investigations. My current chronograph of choice is the CED Millennium Chronograph (www.cedhk.com )selling for only $179 as of November 2003 (generally you should not even bother getting it without the accessory infrared illuminators for about $79 unless you plan to only use it outdoors in full daylight.) The CED Millennium and the required illuminators and tripod can be transported in good sized briefcase or pack.

The chronograph will give you velocity figures, but those figures are of very limited use without a good ballistic program on a computer. Again, many programs have passed through my facilities, but Dexadine's Ballistic Explorer seems to me to stand head and shoulders above any that I have tested - for easy of use and broad application. There are versions which will work in computers from DOS to Windows XP and the price is only about $50. You can get it from Oehler, but I suggest buying it direct from Dexadine at www.dexadine.com because the creator will actually answer your emails and calls - something that very few such makers will do. I found the instructions to be typical of so many products whose instructions were written by someone who know TOO MUCH about the product to relate to the fellow who doesn't yet know which end of the wheelbarrow to hold! But, even I was able to get going with instructions and examples, from the "How To Examples" from Dexadine's own website. You can download a free trial program for testing. This program is absolutely amazing in the things it will do and it covers the parameters so needed, and usually so neglected, for airguns - very close ranges, very low velocities, very light projectiles, etc. . Within an hour I was printing out all the graphs that I could want - even ones which showed the trajectory of a given pellet when fired up or downhill at various angles!! You really should back up the use of this program with the ballistic tables that I review in the Airgun Literature Review section of this website. The ballistic coefficient info, etc. in that reference is invaluable.



should have posted the link....sorry

[Darren Petts]

One question I'd like to find an answer to is how much room to allow for expansion of air before stripping it off. It's no good trying to strip off air that hasn't had sufficient time to expand but equally you don't want to allow sufficient time to let that expansion destabilise the pellet.

Also has anyone experimented with a barrel with holes in? Some expensive target pistols have air vents direct in the barrel whilst keeping a bit of unvented barrel near the end to stabilise the pellet. This in theory seems the best solution though I'm loathed to take a drill to a barrel to test this. Someone suggested spark erroding the holes to avoid damage to the rifling.

Finally the point about varying groups day to day is valid - the slightest variation in prevailing conditions appears to stuff up even the vertical aspect of your groups.

[hareng]

Finally the point about varying groups day to day is valid -

I will agree with that Daz.
Disagree on the 5mm hole, i have even done them 4.76mm leaving at best 0.13mm clearance! A large hole will do absolutely nothing except hide inaccuracies in the design and manufacture.
Also the spacing issue after the muzzle i disagree on without saying anymore.

Lost some power when i vented a barrel in 99 for someone, didnt want to do it but got him off me back. Work quite well when the burrs are eventually removed and has got to be the best form of air stripper, but lots of hassle.

[Steyr]

This post is now available in Hard back collectors edition....

[Professor]

Fascinating information. Thanks for putting it up. I'd started to wonder about the accuracy effects of the silencer on my TX200, though to be honest, the bunnies aren't asking about it. They seem to twitch and fall over. I think I'll try and evaluate group size with and without the silencer though. I wondered about POI effects too, in the sense of trajectory. Might the silencer lower the velocity slightly. Not that a few fps matters much if the pellet hits the right spot, as long as velocity is consistent.

[wornspring]

[QUOTE=hareng]Finally the point about varying groups day to day is valid -

Disagree on the 5mm hole, i have even done them 4.76mm leaving at best 0.13mm clearance! A large hole will do absolutely nothing except hide inaccuracies in the design and manufacture.

Having tried several smaller dia holes and finding odd flyers 1 in 5-10 shots at 55yds I settled on a hole reamed to 7/32"(5.55mm) for .177 cal as an optimal for the stripper at the end of a 32 ml vol Volumetric silencer. I also tried a .25"(commercial silencer size)dia hole. This was a terrible mistake as the blast is not stripped 3"+shotgun style group) . However it works fine for a .22 cal .

[Professor]

Might the silencer lower the velocity slightly?

Surprisingly, it may do the opposite. Having sited in the rifle without the silencer, the POI shifted an inch higher at 26 yards when I put it on.

This is not what I'd have expected - not at all. I'm not sure I believe it, but it happened with a group of five shots. I might test it again - surely it can't be right? Unless maybe, the silencer is allowing suplus energy in the muzzle blast to keep on accelerating the pellet.

[ken s410]

when it comes to power and silencers you will find if you ever have the chance to fire an airgun with and without a suppressor across a chrono that the power will drop VERY slightly maybe few fps which is a very small amount and the quieter shooting of the gun and giving an airgun pellets first few inches of flight in still air can only help the affects of an air weapon

[David Snook]

Professor

It is worth giving the silencer an extended test, it could be skew (pellet deflection within the silencer).

This effect is not limiited to air rifles, it also happens to rim fire rifles too.

It would be nice to think that the silencer was stabilising the pellet and giving a better flight BC that caused the rise in POI, but 1 inch at 26 yards is too extreme I am sure.

If it is skew then you should experience trouble with grouping.

I have not tried velocity checking with and without a silencer, but i am sure that Ken is right.

David

[Professor]

Professor

It is worth giving the silencer an extended test, it could be skew (pellet deflection within the silencer).

This effect is not limiited to air rifles, it also happens to rim fire rifles too.

It would be nice to think that the silencer was stabilising the pellet and giving a better flight BC that caused the rise in POI, but 1 inch at 26 yards is too extreme I am sure.

If it is skew then you should experience trouble with grouping.

I have not tried velocity checking with and without a silencer, but i am sure that Ken is right.

David

Thanks David. I'm still getting used to this gun, and it's been a while since I regularly shot rifles. I'm not a great shot with it, not as good as I used to be with the others. It does seem to behave erratically, but I'm not convinced it isn't wind effects, or my own inconsistency. At times I'll get a three shot group at 40 yards the size of a two pence peice, and at others it will be 2 and a half inches. I'm not used to airguns, I used to shoot .22 rimfire, .22 magnum, .303, and a .58 caliber blackpowder rifle with huge, ounce and a quarter bullets. I've noticed much greater wind effects than with those (it's obvious why, of course). To be honest, the erratic performance is present with and without the silencer - but I will test it again - maybe on a calm day...

Thanks to Ken, as well. That's what I'd have expected, Ken.