If you were machining a crown would you want it square?

[ballisticboy]

Anyway the question was raised in my mind about what would happen if the crown was out of square, well if you can imagine the moment the pellet tail leaves the crown IF the pellet tail is square and the crown is not then air would leave the space created first so throwing the pellet off and creating a flyer, that's the theory...

So following that theory through then surely every pellet would "fly" off in the same direction so if it was small you would never notice this as it would be dialled out when you set up the scope, it would be a constant.

On first thought after reading your post I was thinking the same, that you would get a bias not an increase in dispersion. But, thinking about it that will rely on the pellets being perfect and the C of G being central. Since the C of G is never central I think the relationship between the crown out of square and the position of the pellet C of G offset, which will be different for every pellet, will produce an increase in dispersion. In other words the out of square crown could magnify the effect of an out of centre pellet C of G.
Thats my twopenny worth anyway.

[PhatMan]

Hello to All,

This matter was extensively tested in Precision Shooting magazine in the USA many Moons ago, and the answer is not what most chappies would think.

A .30-06 target rifle was taken, and its particulars established at 100 yards.

Various crown experiments were then made - angled cuts, and then different types of crown damage.

Some of the butchering made your nuts tighten

What was found was that the point of impact changed with each test, but not necessarily the group size compared with the stock rifle.

WRT the wonky crowns, as long as the cut was not seriously wonky - ca. 10 degrees or less, and the rifling at the crown dressed well, then the rifle shot fine.

Similarly WRT crown damage - providing the damage was moderate, and nothing impinged into the bore, all was OK - the 'worst' type of crown damage tested, was a large burr, intruding into the bore this did very nasty things to the group size, as the bullet was damaged by the burr.

Can you relate this center-fire test with air-rfiles & pellets - who knows - it would be very useful if somebody did some experiments

I would suspect that as long as the cut was well reasonably square, and reasonably well finished, and any burrs removed from the rifling at the crown, then the grouping would still be OK - however, I may be talking sh!te - we can only find out by experiment

Have fun & a good weekend

Best regards

Russ

[Lol Moore]

Not sure some got my point, look at this exaggerated diagram to show what I mean for an out of square tail, the release of air at the moment the pellet skirt leaves ideally needs to be an even annulus (doughnut ), out of square its not:

LINK https://photos.app.goo.gl/vRMlofAjBlaUASNw2

in response to your original post I believe the crown being square to the bore is important more for the front of the pellet leaving the barrel, if out of square it would tip as it leaves .

Good point but the outcome should be the same in that square pellets, out of square crown then constant error - not flyers...

Interesting stuff Lol Moore .

How were you checking the squareness of the skirt ? Between 2 ground blocks on a surface plate and eyeball the air gap from the head ? maybe ? ...interested in that . I bet there is some tolerance there .

got any pics of your ins and outs using the rubber stuff ? ( kinda cratex ?) how are the leading edges ?

here's a thought .....wonder what a deliberately wonky crown would shoot like .......wouldn't be hard to find out ......would need to recrown back to square again to check though


Sure the theory about crowns is that it needs to be square or the gas escaping first from the early gap will offset somehow the projectile .

Here's another thought , The lands apply some pressure /contact to the pellet .

If the end is un square , first affected will be the head , do you think the whole pellet could cant in the bore , even before the skirt gets to end ? .Then when the skirt gets there , the same uneven bias is applied and you have a wobbly projectile from the get go .

the tighter the choke , the greater will be the affect .......

does that make sense ???

Praise indeed, I have read some of your stuff on the other side but its almost impossible to follow as a lot is references to photos which I cannot see as I am on the naughty step...would be nice to join in but hey ho

Checking squareness just as you described, three ground parallels and MK1 eyeball with some microscope help, then turn 90 degrees and look again, thought I would see something and maybe be able to measure the gaps with feelers but quick look see yielded nothing - it needs more work.

On in/outs - show me yours I will show you mine To keep stuff square (I supposed a square crown but an off square finishing of the lands would be just as bad) I have made a simple bed on which I lay the barrel I then use a Dremel engraver (which can do 6000spm) secured on the bore centre line with some shaped (on my bench grinder) rubber abrasive burrs to shape stuff finishing with a VFG on a home made mandrel with some Autosol.

Forgot to mention sick of changing chucks so made spiders for both ends of the barrel for machining

LINK https://photos.app.goo.gl/lRS96RsNgf32bkow2

https://photos.app.goo.gl/EPngpqXV2VIrfjGk2

I have not deliberately shot out of square barrels (but read of some who have) but have tried out of square pellets I created with some emery paper - results... have a guess?

Cardews tested this and IIRC found that the gun would still be accurate but the pellets would all leave at an angle to the boreline.

As in my original post, out of square crown with square pellets accurate and predictable

On first thought after reading your post I was thinking the same, that you would get a bias not an increase in dispersion. But, thinking about it that will rely on the pellets being perfect and the C of G being central. Since the C of G is never central I think the relationship between the crown out of square and the position of the pellet C of G offset, which will be different for every pellet, will produce an increase in dispersion. In other words the out of square crown could magnify the effect of an out of centre pellet C of G.
Thats my twopenny worth anyway.

If the out of centre is random then could an out of square crown not amplify or cancel out the error depending on release angle? When all the stars align, out of square with out of CofG - a flyer?

Hello to All,

This matter was extensively tested in Precision Shooting magazine in the USA many Moons ago, and the answer is not what most chappies would think.

A .30-06 target rifle was taken, and its particulars established at 100 yards.

Various crown experiments were then made - angled cuts, and then different types of crown damage.

Some of the butchering made your nuts tighten

What was found was that the point of impact changed with each test, but not necessarily the group size compared with the stock rifle.

WRT the wonky crowns, as long as the cut was not seriously wonky - ca. 10 degrees or less, and the rifling at the crown dressed well, then the rifle shot fine.

Similarly WRT crown damage - providing the damage was moderate, and nothing impinged into the bore, all was OK - the 'worst' type of crown damage tested, was a large burr, intruding into the bore this did very nasty things to the group size, as the bullet was damaged by the burr.

Can you relate this center-fire test with air-rfiles & pellets - who knows - it would be very useful if somebody did some experiments

I would suspect that as long as the cut was well reasonably square, and reasonably well finished, and any burrs removed from the rifling at the crown, then the grouping would still be OK - however, I may be talking sh!te - we can only find out by experiment

Have fun & a good weekend

Best regards

Russ

No sh!te there mate, I will look for that one, not sure the effects of an out of square bullet tail would be as marked on big stuff as the energy involved may damp it out - sure Ballistic boy will tell us?

[ballisticboy]

If the out of centre is random then could an out of square crown not amplify or cancel out the error depending on release angle? When all the stars align, out of square with out of CofG - a flyer?

That is what I am thinking, not just for C of G but for any pellet fault.

No sh!te there mate, I will look for that one, not sure the effects of an out of square bullet tail would be as marked on big stuff as the energy involved may damp it out - sure Ballistic boy will tell us?

There are going to be a lot of differences between bullets from rifles and pellets from air guns. The back end is a completely different shape to start with (cylindrical or boat tailed compared to flared) and the moments of inertia of the projectiles are of a different order of magnitude. The method of stabilisation is different, gyroscopic for the bullets, aerogyro for the pellets. The gas pressure at the uncorking moment is going to be very different. Finally, the basic aeroballistic properties of bullets compared to pellets make the bullets much better equiped to deal with any disturbance on leaving the barrel and thus have a small change in dispersion though you will still get the bias as shown. Pellets are very poorly equipped to deal with any disturbance and could show increased dispersion as well as a bias. Of course, if all your pellets are manufactured to a very high standard and have no or very little assymmetries then you will see a bias but very little change in dispersion.