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On the Bergerbullets.com web site there is an easy to use online calculator that lets you play around with many variables and see what effect they are likely to have on bullet stability.
http://www.bergerbullets.com/twist-rate-calculator/#
There is one thing about it that is puzzling me and I hope someone can say where I am going wrong.
The calculator only asks for muzzle velocity and doesn't ask how far you are shooting, so the stability it gives suggests it doesn't matter if the bullet travels 10 m or 1,000 m. With velocity dropping off with distance, wouldn't this have a significant effect on stability?
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treat the velocity at range as muzzle velocity and you can see whether the round is likely be stable at range ie transonic stability ? or is it that the bullet once stabilised is effectively stable for the rest of its flight? beats meOriginally Posted by Grafter
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Registered User
My Guess is if it's not stable at muzzle velocity it's not going to stabilise so that's what it's based on and how quickly it slows and destabilises depends on other factors
Thanks for looking
Guest
its a very basic calculator and from my experience not truly indicative of a bullet weight/length stability
I have had bullets it says will stabilise all over the shop and key holing and those that it says wont stabilise shoot very nicely
if the combination is anywhere marginal it is best sucking and seeing rather than discounting on the basis of the calculator
Nfn rng w txt spk
I would think so too. Calculated stability does decline with velocity, but other factors such as CG/CP relationship can't be taken in unless they're already embedded in the code, in which case they can only be valid for Berger bullets.
I can't see that this could be a comprehensive calculator.
...history... is, indeed, little more than the register of the crimes, follies, and misfortunes of mankind. (Edward Gibbon: Decline and Fall of the Roman Empire)
Registered User
Kolbe's stability calculator (ex Border Barrels) is still up at:
http://www.geoffrey-kolbe.com/barrel_twist.htm
It's more comprehensive than some. It uses a set of standard bullet profiles, a check-box for type of construction (jacketed etc) and a density.
He reckons that stability improves as the bullet goes down-range (presumably with an implicit "until it hits the transonic region").
AYA SLE 12b, Harrier FAC air, Sako Finnfire .22
Registered User
Thanks for all your thoughts, they inspired me to do more searching on the internet and I found a number of places that give a little more explanation to the factors affecting stability.
I gather there are two aspects to stability - Gyroscopic Stability and Dynamic Stability. From what I have read, Gyroscopic Stability is at it's lowest value at the instant the bullet leaves the muzzle. From then on the bullet spin decays at a slower rate than the velocity and so Gyroscopic Stability actually improves further down the range.
I am still struggling to come to grips with the Dynamic Stability part - the rain had stopped, so I went out and did some real shooting instead
Registered User
Dynamic stability is easy enough if you're happy with hand-waving rather then PhD level (I am). It's positive if the Centre of Gravity is in front of the Centre of Pressure. The usual examples given are shuttlecocks and darts: the weight is mostly at the front and the drag/guidance is mostly at the back, so they don't need to spin to keep them straight. If a disturbance pushes them sideways, the sideways force from the tail-feathers pulls them back straight again. Also think of trying to throw a paper dart backwards. Good dynamic stability is a help if shooting a slug from a smooth bore.
One thing to note is that the degree of stability will change with velocity, in particular it tends to change quite abruptly as you pass through the sound barrier. In that instance the CG stays the same, but the CP moves, once you hit non-compressibility and the conical shock-wave is set up from the nose. That fact killed a few test-pilots when planes started to break the sound barrier "Mach .98...Mach .99... Oh Shi....". To be overly dramatic I could say that negative dynamic stability tries to kill me, because I fly a tailwheel plane. These have the CG behind the main wheels, and when you land you start to get drag from the wheels at the same time as the positive effect from the tailplane reduces because the airspeed is decaying. In the air it's completely stable, but at a certain point during the landing it suddenly starts to behave like a supermarket trolley and wants to swap ends. That persists for a few seconds until the tail is properly down and you can control it again.
I gather that the stability shift is why bullets may fly straight only to tumble as they go transonic, an effect which will be more pronounced for some bullet shapes than others. I think that the dynamics in the transonic region are hard to model, I don't know how good a job the ballistic calculators are going to do.
AYA SLE 12b, Harrier FAC air, Sako Finnfire .22
Registered User
Thanks sackot, I always feel much happier with an explanation of what is going on, instead of just looking at numbers.
So you fly a plane that lands like a supermarket trolley. Do you need a pound coin to take it out of the hanger ?
Nfn rng w txt spk
Whether the stability of a rotating round with a CG close to or behind the CP is upset depends on negative acceleration as well as velocity. The 303 Mk.VII round is a commonly-cited case in point. It was stable throughout practically the whole of its trajectory when penetrating only air - but when decelerating sharply in any target medium, the CG/CP relationship would produce a powerful toppling force if any asymettrical resistance was encountered, producing severe wounds in living tissue, and folding or snapping the bullet in (eg.) timber or sand.
...history... is, indeed, little more than the register of the crimes, follies, and misfortunes of mankind. (Edward Gibbon: Decline and Fall of the Roman Empire)
Registered User
Grafter,
I wouldn't worry to much about the gyroscopic stability factor, Sg.
I shot the 0.308 155 gr. Lapua Scenar from a 14 twist 0.308 Winchester barrel (Sg ca. 1.1) at 3050 fps, and never had a stability problem in the ten years I shot that load - all the way to 1000 yds., and at times with temperatures below zero.
The dynamic stability, Sd, of the projectiles we are talking about, increases with range - the rotational velocity decays much more slowly than linear velocity, so the dynamic stability increases.
have fun
Best regards
Russ
Nfn rng w txt spk
That's a pretty good point, and completely defeats the idea of using the residual velocity at a downrange distance as the MV for the Berger calculator, unless you calculate and substitute the shorter effective twist rate represented by the retained angular velocity - which I think may be difficult?Grafter,
I wouldn't worry to much about the gyroscopic stability factor, Sg.
I shot the 0.308 155 gr. Lapua Scenar from a 14 twist 0.308 Winchester barrel (Sg ca. 1.1) at 3050 fps, and never had a stability problem in the ten years I shot that load - all the way to 1000 yds., and at times with temperatures below zero.
The dynamic stability, Sd, of the projectiles we are talking about, increases with range - the rotational velocity decays much more slowly than linear velocity, so the dynamic stability increases.
have fun
Best regards
Russ
...history... is, indeed, little more than the register of the crimes, follies, and misfortunes of mankind. (Edward Gibbon: Decline and Fall of the Roman Empire)
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