AR-15s and Primers

[Walkalong]

This looks like a gun problem more than a pressure problem, could be both, but, as posted, you can't read pressure from primers. You can see issues from what a primer looks like, but by the time that happens you are well over pressure.

 

[kwg020]

I looked at some of my old data, and I was piercing WSR at 24.0 grains with a 62 grain bullet. Manuals are a guide, but you need to look at what is happening in the real world, and work off that.


Armalite M15A2 Carbine 16.5" 1:9 Chrome Lined barrel

69 Sierra 24.5 grains AA2520 LC mixed cases WSR (Brass) OAL 2.25"
23-Mar-05 T = 65° F

Ave Vel = 2629
Std Dev = 34
ES = 92
Low = 2594
High = 2686
N = 10

good group but 3 pierced primers!


62 FMJBT 24.0 grains AA2520 lot 4991 LC mixed cases WSR (Brass) OAL 2.25"
11-Sep-06 T = 89 ° F

Ave Vel =2668
Std Dev =34
ES =136
Low =2606
High =2742
N =28


good group but still pierced primers!

I have 2 Armalite barrels. Both seem to have tighter chambers that the NATO standard. They make for accurate rifles but you really have to watch for pressure signs. My guess is the OP has either a tight chamber or a longer firing pin on his AR and is consequently piercing primers. 2520 is a great powder but pressure signs still need to be monitored.

kwg

 

[Driftertank]

I would point out that military pressure specs likely assume a crimped primer.

 

[MistWolf]

This looks like a gun problem more than a pressure problem, could be both, but, as posted, you can't read pressure from primers. You can see issues from what a primer looks like, but by the time that happens you are well over pressure.

The AR needs hard primers to keep primers from flowing back into the firing pin hole and due to its floating firing pin.

The only way a primer crimp makes a difference in handling pressure is if the case isn't being held against the bolt face at the time of ignition.

 

[denton]

The AR needs hard primers to keep primers from flowing back into the firing pin hole and due to its floating firing pin.

The only way a primer crimp makes a difference in handling pressure is if the case isn't being held against the bolt face at the time of ignition.

Exactly.

 

[HankC]

Do you have a light hammer spring? When there is not enough force behind the firing pin the primer cup could force the firing pin back, "fire form" into the firing pin hole and bulge out or cut the primer at the hole like you have in the picture.
"The AR needs hard primers to keep primers from flowing back into the firing pin hole and due to its floating firing pin." But then 7.62x39 primers are too hard that many ARs need a heavy hammer spring and enhanced firing pin to ignite the primers! I would think regular small rifle primers are sufficient to handle up to max load. Folks load over max do not always experience "pierced" primers. I have 223 range pick-ups show heavy ejector marks flow into ejector hole indicating loaded to very high pressure but primers are fine. Your brass do not even show the ejector mark much.