do you crimp for a 308 semi auto?


April 5, 2010, 07:38 PM
I've been hearing people say it's a must put some type of crimp on all rounds going through a semi-308 b/c when the bolt picks up a cartridge from the mag and slams it into the chamber, it can set the bullet back far enough to cause extreme pressure and possible chamber rupture. Others say don't do it, it will kill your accuracy, and is only necessary on machine guns. What's your experience with this?

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April 5, 2010, 07:52 PM
Not necessary.

I have had precisely one semi-auto rifle that telescoped its bullets into its cartridge case: it was one of the "Unibrow" Century FAL rifles. It would telescope them even with crimped military ammo, three quarters of the cartridges in a magazine would have this problem.

M1, M1A, all other FAL rifles, not necessary.

April 5, 2010, 07:53 PM
I use a moderate roll crimp. :evil:

April 5, 2010, 08:07 PM
From the experts at Sierra.

Neck Tension

When we stop to consider the vigorous (read, downright violent) chambering cycle a loaded round endures in a Service Rifle, it becomes pretty clear it suffers abuse that would never happen in a bolt-action. This is simply the nature of the beast. It needs to be dealt with since there is no way around it.

There are two distinctly different forces that need to be considered: those that force the bullet deeper into the case, and those that pull it out of the case. When the round is stripped from the magazine and launched up the feed ramp, any resistance encountered by the bullet risks having it set back deeper into the case. Due to the abrupt stop the cartridge makes when the shoulder slams to a halt against the chamber, inertia dictates that the bullet will continue to move forward. This is exactly the same principle a kinetic bullet puller operates on, and it works within a chamber as well. Some years ago, we decided to examine this phenomenon more closely. During tests here at Sierra’s range, we chambered a variety of factory Match ammunition in an AR-15 rifle. This ammunition was from one of the most popular brands in use today, loaded with Sierra’s 69 grain MatchKing bullet. To conduct the test, we chambered individual rounds by inserting them into the magazines and manually releasing the bolt. We then repeated the tests by loading two rounds into the magazine, chambering and firing the first, and then extracting and measuring the second round. This eliminated any potential variation caused by the difference between a bolt that had been released from an open position (first round in the magazine) and those subsequent rounds that were chambered by the normal semi-automatic operation of the rifle. Measuring the rounds before chambering and then re-measuring after they were carefully extracted resulted in an average increase of three thousandths (0.003") of forward bullet movement. Some individual rounds showed up to seven thousandths (0.007") movement. Please bear in mind that these results were with factory ammunition, normally having a higher bullet pull than handloaded ammunition.

To counteract this tendency, the semi-auto shooter is left with basically two options: applying a crimp or increasing neck tension

April 5, 2010, 08:15 PM
I measure my necks after sizing, and they all tend to be .004 smaller than the projectile. Think that's enough tension?

April 5, 2010, 08:47 PM
I am on the third match barrel on this M1a.

I have never crimped a match bullet after I found out that crimping match bullets swages them in the middle.

If you have sufficient neck tension, and even a cheap die such as a Lee Standard Sizing die will give sufficient, you don't need to crimp/

April 5, 2010, 10:10 PM
Embrace the Crimp!

April 6, 2010, 10:15 AM
I measure my necks after sizing, and they all tend to be .004 smaller than the projectile. Think that's enough tension?

Yep, you will be fine. The only cartridges I would crimp would be heavy recoiling large bore cartridges, and cartridges that feed from an under-the-barrel tube magazine.


April 6, 2010, 11:02 AM
How about recoil forces (definitely more in 308 than 223 of course).... does the bottom round in the mag, subject to 19 recoil cycles in an M1A, FAL, etc, have the bullet move as a result?

April 6, 2010, 11:52 AM
I always crimp for semi-autos. I don't know if it's necessary, but it gives me peace of mind.

April 6, 2010, 11:56 AM
I crimp for semi too. I load for several different rifles and don't know which one I'll be shooting so I always crimp.

April 6, 2010, 04:32 PM
I have never needed to crimp for my M1A.

April 6, 2010, 11:23 PM
I have not crimped for my 308 M-1 or '06 M-1's unles there is a crimp groove. Since I shoot a lot of SMK bullets with no crimp groove I haven't crimped in many years.

I've never crimped a 223 for my AR's. Again I use SMK's.

April 7, 2010, 07:32 AM
I used to never crimp for my semis, until someone posted the Sierra advise shown above. So, I started checking, and sure enough, some of the bullets were slipping out when the bolt slammed home. So, I bought a Lee FCD for my 223 and 308 guns. Just a light crimp holds them in place now.

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