Worn / Damaged Crown

As told to me, the pressure wave which both precedes and follows the bullet needs to expand smoothly and symmetrically at the tip of the barrel. A ding on the crown can give an assymmetry in the suddenly-expanding pressure wave. This in turn can act on the bullet and disturb it as it exits the barrel. Thus, the bullet doesn't touch the ding, but is nonetheless affected by the ding. Now, if bullsh*t was told to me, then I do apologise for repeating it here and would appreciate to know the truth. But, as I stated above, it is simplicity itself to burnish a ding out of the crown.

I suspect that the answer is in the CONSISTENCY of the "crown".

If some dopey digger has used a dirty pull-though poorly for some time, there will be "wear" to both the ends of the lands and the grooves.

Thus, the gases exiting at "ludicrous speed" from behind the bullet will generate an "eccentric" blast cone that briefly races past the bullet as said projectile exits the muzzle. "Gas erosion" of the "crown" is a factor.

NO bullet is perfect. Thus the bullet will be precessing (making a small spiraling motion) as it leaves the muzzle. Enveloping it with an eccentric shock wave is probably not helpful.

HOWEVER, I suspect that all of the preceding applies MUCH more to the dreaded boat-tails than "proper" Mk7 bullets. That little cone of a boat-tail means that any following gases have a GREATER time during which they can bear on the bullet. Many hard-core bench-rest types use flat-based bullets for "short ranges" and only reach for the boat tails when working 500-1000 yards. Additionally, forming a boat-tail involves the use of another die-station in the machinery and thus adds another process that can result in tiny, but significant eccentricities in the core and particularly the jacket, and ultimately, balance. Again, this is most noticeable at "shorter" ranges.

Finally, if your trusty Lee Enfield barrel has been fed a steady diet of Cordite-fueled Mk7 ball, its throat WILL be eroded to some extent. Gas "blow-by" similar to that which occurs at the muzzle WILL also occur briefly at the throat upon ignition. Furthermore, as "sporting" boat-tailed bullets have a "solid" base, they are less likely to "bump-up" when kicked in the backside by many thousands of PSI. Think of using such boat-tails as applying a tiny, circular plasma cutter to the inside of your barrel.

Quote:

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Somebody want to explain these results? How can a barrel with a crown that has been absolutely destroyed shoot under ¾ MOA? Beats me. I saw it and I still don’t believe it. Well I do believe it because I saw it, but I can’t explain it. And since I can’t explain it I am going to continue to insist on having as perfect a crown as is possible.

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That sums it up.

Quote:

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Originally Posted by Surpmil

Somebody want to explain these results? How can a barrel with a crown that has been absolutely destroyed shoot under ¾ MOA? Beats me. I saw it and I still don’t believe it. Well I do believe it because I saw it, but I can’t explain it. And since I can’t explain it I am going to continue to insist on having as perfect a crown as is possible.

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Now that we've been all through this, that statement seems like a bit of "Pig headedness". I still believe that when producing a barrel, the end result should be as cosmetic as possible...clean and sharp.

An interesting thread, think there will be a few on either side of the fence on this one, I,ve only re crowned a few full bore barrels but this was due to visible damage etc and the owners request, I've done plenty of my own and heaps of Ruger 10/22 and similar barrels after length reduction and cut a 11 degree crown and had no complaints, (yet).

After working on very big oil fired boilers, that require atomising steam for a cleaner burn, a vital bit of kit is the swirl plate which..... anyway when blocked or semi blocked can have a dramatic effect on efficiency, so I,m in favour of clean sharp ( like BAR) machined edges (but that also goes back to my apprenticeship)

Thanks for bringing this up Alan. I've always heard how important this was, even did the brass screw and valve compound jobby on a couple rifles... Didn't notice anything except an incongruent look in the finish of muzzle :) still not sure about the stressed importance.
Cheers