The effect of crimping on accuracy has been argued for years, especially
among those interested in precision rifle shooting, commonly called “Bench
Rest.” I did that for awhile, although not in competition. My own opinion is
that crimping can affect the accuracy of cartridges when you are speaking
of shooting 5 shots into basically one slight enlarge hole at 100 yards with
a rifle. I have tried it both ways in both the .222 Remington (Varmint or Light-
caliber class) and .308 Winchester (Heavy varmint or caliber class). I always
got better results with no crimp, where every other factor, including case-neck
turning, were the same.
That said, for any field use, and most certainly in ammunition designed for the
rigors of military combat, including shipping, handling, loading feeding devices with
or without loading machines, carry in ammunition pouches and even rough handling
of the loaded weapon, I would demand crimped-bullet ammunition. Proper crimping
can produce very accurate ammunition in every sense of the word, including normal
match position target shooting, but I still believe that when the ammo and the rifle
can be handled with extreme care, with no problem, a proper bullet fit in the case
neck, with no other bullet crimp is slightly better. That is just my own opinion based
on my own shooting experiences.
Regarding crimping and “bullet pull,” testing is often done for weight of pull with crimps
or lack of crimps in mind, mating of inside case-neck diameter and bullet diameter, and, yes,
even for external and internal neck seals. Bullet pull is very important to not only accuracy,
but also to maintaining safe and proper pressures. That is even true in extreme circumstances
of hunting and combat. A loose bullet pushed back into the case reduces the size of the
powder chamber and raise pressures.
As already said, in revolvers the crimp is necessary to keep the forces of inertia from recoil
forcing the bullet partially out of the case in the unfired chambers, causing a lockup of the
cylinder. In an auto pistol, crimping is still important (I always favor a taper-crimp over a roll
crimp due to the aforementioned headspacing considerations). Even in a magazine, a hot
enough load can force a bullet, and even the whole loaded cartridge to move forward in the
magazine with sufficient force to dent the front of the magazine box from within. I have witnessed
this on Luger magazines and even on Colt/Browning style .45 magazines when fired a lot with
FMJ ammunition in full loads.
As any ammo factory guy knows, and most reloaders now, it is never just a matter of sticking
a primer in a case, putting in the powder, and seating a bullet with not other considerations.
Crimp or no crimp can be very important either way.