Got it,
So whats best here...taper crimp or roll crimp?
1- No crimp at all IF that is possible. This might be a possibility IF there is enough case neck "tension" to hold the projectile against all the feeding and chambering forces and also keep the projectile from being forced into the case during its time in the magazine. Case neck "Tension" is here defined as the number of 0.001" that the neck is UNDER the actual projectile major diameter that act to "hold" the projectile when it is pressed into the case neck. Just so the chamfering is done well this is the method that is least likely to screw up a projectile that is otherwise a good projectile to start with.
2- A roll crimp can be considered IF there is a cannalure in the projectile to crimp it into. It is important that the case length be uniform if a roll crimp is employed as non-uniform case lengths result in non-uniform roll crimps because the
end of the case is turned into the crimp groove by a shoulder within the die. This is in my opinion the kind of crimp with the best "holding" ablity. Try looking into a LEE Factory Crimp die which crimps in from the side and which produces a similar type of crimp which is much less dependent on case length. If there is no cannelure on a projectile using a roll type crimp can seriously damage that projectiles grouping potential.
3- A taper crimp is literally a tapered die which pushes in slightly against the end side of the case neck and tends to just put a slight crimp in the neck which is somewhat insensitive to case overall length. It's considered to be a lot more "gentle" on non-cannelured projectiles and is the preferred method of crimping them IF a crimp needs to be done.
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For service rifle I would myself consider AT LEAST a 2.5-3 thousands under projectile diameter and up to about a 3 to 4 thousands under projectile diameter cartridge case neck tension with no crimp at all as a first choice in ammunition set up for match grade accuracy. I would then do a test series with ammo with already measured and known overall lengths by chambering a round then dehambering it and see if the chambering resulted in an overall length (OAL) change, then by firing one round then dechambering the chambered following round to see if the cartridge overall length had changed at all. Etc. IF the ammo is maintaining its OAL after this testing I would just load up a lot with the same exact components and go compete without any crimp at all. If the projectile OAL's ARE changing then this may not be the method of choice for your ammunition. The OAL's
MUST be demonstrated to
not change if no crimp is to be employed and neck tension only is to be used in a service rifle.
When loading military FMJ or other cannelured projectiles for any kind of “social” circumstance I always crimp them into the projectiles crimp groove as I do NOT want those projectiles moving NO MATTER WHAT and I feel I may as well take advantage of any extra powder burn uniformity that a firm crimp can help provide in exchange for the tendency of that same crimp to screw up the uniformity of a projectile to some degree and the resulting degradation of precision grouping that it leads to. Use no more roll crimp that is actually necessary and guard carefully against crimping so much that the projectile can become distorted.
Match grade projectile VERY OFTEN have no crimp grooves. Projectile makers literally cringe when people start talking about crimping those carefully manufactured match projectiles. BUT, SAFETY MUST COME AHEAD OF ALL ELSE and IF one has a good grouping load and relatively low neck tension what can be considered and done is to then apply a judicious and somewhat light taper crimp to that low neck tension ammo and let that mild crimp help act to keep the projectile from moving and screwing up the precision of the load and also possibly creating a safety hazard. It’s always nice to have a very uniform cartridge overall lengths but in the case of the use of a taper crimp one can be a lot less picky about slight differences in length as the taper in the taper crimp is very modest and relatively insensitive to slight changes in overall length. This type of crimp strategy has at times been employed in an attempt to have a load worked up for a bolt action also be useable and safe to also use in a semi-auto like an M-1A so one does not have to have two loads which for two rifles but instead can use one load for two rifles.
There’s a lot to think about with crimping and I hope this usage of board bandwidth goes to the point of what is being inquired into.
I feel there are a lot of members here that can share their crimp experiences with you. Please do not hesitate to post again with any questions or concerns you may still have about this issue and someone I’m sure will try to address your concerns.
