BARREL EXPANSION or CONTRACTION??

[camac]

IMO a better quality barrel displays fewer quirks from heating up, whether from loosing stiffness or from dimensional changes. By better barrel I mean one with no localized residual stresses from the barrel making process, with the bore centered in the barrel and everything nice and straight.

Rust, I agree, the quality of the bore does seem to make a difference. My worst fouling and worst looking (borescope) barrels seem to increase velocity the most. I am not sure if they have barrel stress issues or are off centre (they measure good) but they are cerainly "rough cut or lapped". Maybe the copper fouling/friction theory does come more into play?? These barrels respond like this even when the barrel is fouled before starting the coldbore shot.

 

[Rust]

Reduction in Modulus of Elasticity within the temperature ranges discussed is not significant. Up to about 600°f there is less than 10% modulus reduction in common barrel steels

If a barrel were a static system, then perhaps only a large change in Youngs Modulus of Elasticity would matter. But since a barrel is a dynamic system one has to consider how a lesser reduction would interact with everything going on in a barrel. Would a 1% change in elasticity change the barrel harmonics to the point the bullet exit is no longer at a null point in the vibration pattern? Does it happen at 2%? Maybe 3%. Do other factors such as the degree of cocentricity of the bore to the barrel have any effect on the possible interaction?

And given unequally distributed residual stresses, as the barrel heats, do those areas with unequal stresses react uniformly, does their particular elasticity change in step with the barrel as a whole or not. And how does this affect harmonics?

Easy way to tell if there are significant residual stresses. Is the POI of the first cold bore shot significantly different from subsequent shots. Of course it all depends on what you want to consider significant, to a benchrester throwing a shot 1/10th moa can be huge, while a hunter might consider minute of whitetail acceptable.

I'll grant that my thoughts on some of the effects discussed are theoretical. But unless there is repeatable empirical evidence that a change in elasticity has no effect what-so-ever on the dynamic system of a rifle, or at least no significant effect, I won't dismiss it just yet. Shouldn't have much effect if any on a good barrel, might have some effect as quality decreases.

 

[Rust]

Some barrels do show an increased velocity with increased fouling. he increased fouling makes the bullet a little harder to start down the bore and travel down the bore which allows for more powder burn per amount of distance traveled down the bore. The problem is that this makes the the initial chamber pressure is higher which could cause difficulties with hot loads. Sort of like firing a heavier bullet with the same charge as a lighter bullet. I should point out that heavy uniform fouling will act more as a bore constriction than a friction modifier.

Some of the better (IMO) match barrels have a taper lapped into the bore, with a decrease in bore diameter of .00015" to .0002" towards the muzzle. The initial chamber pressure stays low, but the gradual taper make for a higher pressure (compared to a bore of unifomr diameter) as the bullet travels down the bore for a little more muzzle velocity. Perhaps the best example is not a rifle but the older Colt Python revolvers which had a definite taper to the bore. They were noted for being both fast and accurate.

 

[camac]

A rapid increase in heat applied from the bore will give the greatest contraction of bore diameter, and a thicker walled barrel will show a greater degree simply because of the higher differential temperature from the bore to the outer surface. Under any normal firing conditions I'd venture from .0002"-.0003". Might get more tyring to imitate a machine gun.

IMO a better quality barrel displays fewer quirks from heating up, whether from loosing stiffness or from dimensional changes. By better barrel I mean one with no localized residual stresses from the barrel making process, with the bore centered in the barrel and everything nice and straight.

Rust - Just had another thought we often think of the lighter barrels as showing "more" effect of rapid fire and barrel heating. They seem to rise more under string firing. This sort of also flies in the face of my original theory too. Barrel contraction must only play a small role in pressure build up (velocity rise") as it should be worse in large diameter barrels, not the other way around.

Also on the fouling - I agree but a cold barrel with fouled bore still rises quickly in velocity with temp as well.

Whilst I have no doubt all of the above are happening I think the most significant effect is not just fouling, not just bore restriction then expansion, and not just barrel warping. My worst barrel for doing this is the one that collects most fouling but it also rises significantly from cold to hot in velocity even when heavily fouled.???

The fouling/friction with temp theory could possibly explain it.
ANy ideas on how to test??? A great PhD project....

Thanks for the great input all.

 

[Tozguy]

and thats why I think the increase in velocity from barrel heating up is largely due to powder burn rate increasing with barrel temp.

 

[Tozguy]

In a recent daily bulletin, it was reported that no adjustment to loads was required after coating a bore with nitride. Logically, the coating restricts the bore slightly and must reduce friction significantly but still no need to adjust load. Is this another sign of how little these factors affect velocity?

 

[spclark]

Not sure there's enough experience with nitrided barrels yet to put a reliable answer to your question. I know I don't have any yet, but that may change soon.

My experiences with moly indicated a need for small bumps in charges. When I switched to hBN I found few loads needed alteration, whether using my home-coated bullets or those supplied coated from SSS. Never used DANZAC.

Nitrided bores may behave quite differently than uncoated with any of the materials now used to coat bullets. The degrees of change - thickness of nitride coating, reduction of friction - may be insignificant when compared to uncoated bores with coated bullets.

I'll take comments from John Whidden as gospel though, any time!