Cratered primers on FGMM fired from my POF P308. Is that normal?

[hondakilla98]

I'm getting ready to start reloading for my P308. I wanted to compare the headspace from a fired round and an unfired round from the same box of FGMM. That's when I noticed all the primers were cratered, and a little flattened. And quite a few have ejector marks. This is old white box FGMM, i'm just wondering if this has happened to anybody else.

Here are some pics.

 

[ChrisGarrett]

Is your firing pin hole oversized?

Chris

 

[hondakilla98]

Is your firing pin hole oversized?

Chris

Good question. I'll take a pic and you can tell me.

 

[djarecke]

Mine were cratered but no ejector marks. They chrono'd at 2775 FPS out of a 24" barrel. They seem a bit hot...

 

[rduckwor]

Any other pressure signs?

A couple of those appear to have a pretty solid ejector mark. As for the primers, I wouldn't worry too much about them. Looks like flow into a large FP hole. Fed primers are typically thought to be "soft".

 

[JRM83]

Could also be a headspace issue as well. Most factory ammunition is sized smaller than the minimum SAAMI base to shoulder length to allow chambering in a variety of firearms. I had a CMMG Mk3 .308 where the chamber was .006 above SAAMI min and factory brass always showed excess pressure signs on the first firing because the undersized factory brass had to stretch .010 to fill the chamber.

On the other hand, I have measured the fired brass coming out of two different POF P308s and found that the POF chamber was cut close to SAAMI min on both so there shouldn't be much stretching unless your chamber is larger than usual.

 

[6brshooter]

Looks to be over gased. My DI ar10 308 and 6.5 cm did that until i put a jp adjustable gas block on them. Over gased causes it to start unlocking prematurely, ejector swipes were common on mine so i downloaded the ammo. After putting on the jp block i was able to pick up close to 100 fps extra without getting ejector marks.

 

[hondakilla98]

Looks to be over gased. My DI ar10 308 and 6.5 cm did that until i put a jp adjustable gas block on them. Over gased causes it to start unlocking prematurely, ejector swipes were common on mine so i downloaded the ammo. After putting on the jp block i was able to pick up close to 100 fps extra without getting ejector marks.

I'll try the other gas settings and see if I get different results. I have 3 positions on the gas piston setup. Regular, suppressed, and off.

 

[BCP]

Are these the only rounds you've fired through it? Without some sort of baseline to go off of I don't know what to tell you.

 

[JGorski]

Is your firing pin hole oversized?

Chris

That's what I was going to say.

 

[Vio515]

I agree on the large FP hole seems a tad oversized.

Also, for my own edification on gas guns, the primers don't look too flat or over-pressured but those ejector marks are pretty significant. How does the bolt unlocking too early cause that?

Thanks for the knowledge.

 

[FLIGHT762]

My 1993 era SR-25 does the same thing with Federal Factory 168 GMM. There are several factors, Federal brass is soft and more prone to showing ejector swipes. Gas guns are hard on brass and gas guns usually cannot handle the factory loads that bolt guns can. You can check your firing pin hole with a pin gauge. The hole shouldn't be over .065". In reloading for 308 gas guns, you do need to stick with powders in the medium burn speed and stay away from powders any slower than Varget.

I mitigated the issue by using harder brass, loads that are a couple of grains of powder lower than my bolt gun loads and I have a Tubbs Carrier weight System (CWS) that can help a bit. Tubb Precision AR-15 Carrier Weight System

When My SR-25 was new, it would blow a couple of primers with Factory 168 GMM. It settled down some when it broke in, but I did have issues with some Hornady 168 TAP ammo.

When Tubbs and other were using the SR-25 / AR-10's for long range, the found moving the gas block forward helped.

I have a GAP 10 t308 Win. that was built last year. It also is a bit finicky with hot loads and will also beat the brass badly.

 

[6brshooter]

By moving the block forward u decrease dwell time, pretty much the same as decreasing gas pressure that an adjustable gas block does. Decreasing gas or adding more weight to the buffer/carrier helps keep the bolt locked longer. That slight increase in bolt lock time keeps the case from streching and slamming into the bolt face. I had no problem running h4350 in my 6.5cm gasgun, once i installed an adjustable gasblock. The one problem with adding weight to the buffer is that it makes the rifle recoil a little more and when bolt returns to battery the whole rifle jumps more. An over gassed ar tends to beat up all parts involved in the cycling process, including brass.

 

[hondakilla98]

Are these the only rounds you've fired through it? Without some sort of baseline to go off of I don't know what to tell you.

I've put other ammo through it, unfortunately I wasn't looking at the primers. I'll be hitting the range to do a ladder test soon. I'll take some other factory ammo and see if any of it gives me similar results.

 

[Aimsmall55]

That's what I was going to say.

+1 on this. Or your pin has too much "push" behind it.

 

[6brshooter]

Primers look pretty normal, much like a non bushed rem, savage, or fn. The primers are still rounded at the edges and show very little flattened area. The suppressed setting would be closing gas flow down, i'd try that. Not familiar with pof pistons, but carrier weights or a heavier buffer might be the only option. With 155 in my ar10 i could only get 2750 before ejector swipes. After installing a jp block i could get up to 2840 without swipes. Since it's a floating pin setup kinda hard to limit pin strike. If it was a piercing problem I would suggest checking protrusion of pin. The bolt hole could be bushed but that has nothing to do with the swipes. Make sure the chamber is completely oil/solvent free, a oily chamber can cause ejector marks on mild loads.