Worn / Damaged Crown

[Bruce_in_Oz]

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.