RFB(and other Semi-auto rifles) to crimp or not to crimp?

[350 Mag]

I used my RFB for hunting this Fall.

I noticed that after a round had been chambered and ejected(un-fired) that the OAL was increasing?

I confirmed with calipers this morning that indeed loaded rounds that were cycled through the chamber did in fact increase in length by .030. I didn't keep track of the number of times they were cycled.

When I loaded my hunting rounds I lightly crimped with Lee FCD....but now I think that a little heavier crimp appears to be justified?

It seems odd to me...but I would have expected the opposite(shorter OAL)?

 

[canucklehead]

Definately crimp. I use the Lee factory crimp on all my .308 reloads and have never had issues in Semi's with increasing OAL. Even in my RFB!

 

[Claven2]

I haven't been crimping and it hasn't been an issue for me in my gun.

 

[cancer]

You should always crimp anything being chambered into a semiauto. Some guys will say it doesn't matter, but why risk it?

In your case, rounds that are coming out longer than they go in suggest that the bullet is moving into and out of the neck during cycling. If that bullet goes too far back and you pull the trigger on it, you could have wandering vertical strings in the best case and a catastrophic Kaboom from an overpressure cartridge in the worst case.

 

[lister]

I would have thought that it'd be a shorter OAL, not longer, but yeah, heavier crimp for semi-autos.

 

[geclark]

crimpin ain't easy.

 

[X-man]

I crimp for semis. Even factory hunting ammo. Most surplus/military FMJ stuff is already sufficiently crimped. Many moons ago I made the mistake of shooting some 45gr Winchester white box .223 in an AR. The light crimp on the factory ammo and the nose profile caused feeding problems and bullets to be set-back into the case, potentially raising chamber pressure and definitely causing ejection problems. After figuring out what was going on, I forevermore made sure to crimp any ammo being fed to my semis, especially ARs.

 

[Ar180shooter]

I have used uncrimped ammo in my Swiss Arms without issue, but I now crimp everything, just to be sure.