Need help identifying this pierced primer problem

[Ultraman550]

Specs of rifle:

1:7 twist Stag Arms
16" Carbine length
AAC 7.62 SDN-6 QD suppressor

Spec of ammo:

LC 5.56 brass
1x fired (this was first reload)
case trimmed to 1.750
OAL of bullet/case 2.250 (magazine length)
23.5 grains of H335
69 SMK
Fiocchi primer small rifle primer
*** primer pockets swaged with Dillon 600 super swager
*** to my knowledge the primers went in smoothly, not easily but it was a nice snug fit
*** flash holes deburred

I had shot this exact combo out of a 1:9 twist, 16" carbine length Stag Arms barrel before and never once saw this happen. I didn't have the suppressor on though. As you can see it happened to a bunch but not all. I checked the cases and there were no signs of overpressure. I cut one of the case in half that had a pierced primer and there was no case head separation, primer pockets didn't seem to be stretched out to the point of the primer being able to come out without a daccaping rod. I also checked the firing pin and it looks ok to me, no pitting and does not extend too far from the bolt. Out of 37 rounds there were these 11 that this happened to. The other primers that were not affected looked fine with no signs of over pressurization, no cratered primers. Also in the second picture its kind of hard to see but there is a little protrusion of what appears to be metal coming out of the primer. Could the heat have melted the primer then when it cooled this is what happened?

 

[thrusty]

I had shot this exact combo out of a 1:9 twist, 16" carbine length Stag Arms barrel before and never once saw this happen. I didn't have the suppressor on though.

Read what you just wrote and you may find the answer you seek grasshopper.

 

[Ultraman550]

Honestly and I'm not just saying this but I did think that for at least a second. I loaded up 10 with 23 grains and 10 with 23.3 to see if I need to back it down a bit. I also loaded 10 with 23.5 and CCI BR primers, 10 with 23.3 and CCI BR primers and 10 with 23.0 and CCI BR primers to see if different primers would be better but I have not tried them yet. I think in general I need to back it down and start over.

 

[thrusty]

Not necessarily creating pressure....but it will effect the gas system of the rifle, which in turn, causes all kinds of possible issues if you're not compensating for it (ie: adjusting gas block or buffer weight).

Shoot the gun without the suppressor on with the same ammo and see if the issues are there. Give the gun a good cleaning too before you try it without the suppressor.

 

[bigedp51]

The majority of AR15 rifles are over gassed, meaning the bolt can move to the rear while there is still residual pressure in the barrel. This pressure forces the primer to the rear more than normal and turns the firing pin into a cookie cutter and it punches the center out of the primer. With the suppressor added there is even "MORE" residual pressure in the barrel and the primer moves further to the rear.

There is a reason why the military crimps their primers, the rifles are gas operated and they have longer headspace setting than civilian rifles. On a "uncrimped" primer you pull the trigger and the firing pin hits the primer and drives the case forward until the shoulder of the case contacts the shoulder of the chamber. The cartridge goes "BANG", the case grips the chamber and then the primer moves to the rear until it hits the bolt face. Long story short, after the firing pin hits the primer, and puts a dent in the primer and then chamber pressure forces the primer back "OVER" the firing pin for a double whammy and knocks the center out of the primer.

Head clearance is the air space between the bolt face and the rear of the case, over resizing the case (excessive shoulder bump) creates "MORE" head clearance and the chance of a pierced primer.

Below, watch the primer move to the rear when the cartridge is fired and then watch the brass stretch to meet the bolt face. Over resizing the case, the type primer and residual pressure in the barrel will cause the center of the primer to be punched out like a cookie cutter.

If your going to use your suppressor then use the thickest primer cups at .025 thickness below and added insurance would be the CCI #41 military type primers. Next I would get the Hornady cartridge case headspace gauge and measure your shoulder bump carefully, and not use any cases with loose primer pockets.

 

[Ultraman550]

Wow, awesome info. Thank you very very much.

 

[farmer7]

Not necessarily creating pressure....but it will effect the gas system of the rifle, which in turn, causes all kinds of possible issues if you're not compensating for it (ie: adjusting gas block or buffer weight).

Shoot the gun without the suppressor on with the same ammo and see if the issues are there. Give the gun a good cleaning too before you try it without the suppressor.

I know you said not "necessarily creating pressure" but just an FYI I just chronographed my .308 20" Tikka with and without the supressor and was surprised at finding my average without the supressor was 2630fps and with averaged 2663fps. An extra 33fps is going to produce more pressure, in my rifle anyway.

Dont know if that is normal or does it come down to the supressor? And I have no experience of semi auto centrefires as we're not trusted with them over here!

 

[RugerSlinger]

I would say when you bumped the shoulders back you bumped them too far. I did this accidentally in one of my ARs and it did the exact same thing.

 

[sinister]

It could be a combination of BOTH shoulders set too far back (check with a Stoney Point gage) AND the use of a 7" Carbine gas system with the suppressor.

23.5 isn't particularly hot.

What weight buffer are you using?

You might want to use a heavier buffer (H3 or XH) and/or add a Carrier Weight System to extend your dwell time due to the short gas system and the increased back pressure from the suppressor.

 

[bruddah]

Excellent information here!