Dirty Dirty Russian (Ammo)

I have a 16 inch x39 build I completed a year or so ago that has something like 1500 rounds of steel case through it and has run flawlessly. It has been great. And with Hornady Black is actually an MOA or better rifle.

So I decided to build an 11.5 gun. Both are CLGS, using an A5H2, RCA BCG (which uses a true x39 bolt and not a hogged out 5.56 to provide good firing pin protrusion) and a LaRue MBT (which has a full power trigger spring.) The only difference was the gas port size. The 16 inch is .093 and the 11.5 (which arrived woefully undersized at .078) I opened up to .089 to start, figuring decreased dwell time might come into play so I wanted some room to grow if need be.

Off to the range a few weeks ago along with a SICO Chimera for fun. Using a variety of steel case to include Tula, Red Army and Barnaul. First five rounds go click but no bang. Hmmm. Primer strikes are deep and even. Load up another mag. Finally get a few to go bang but more that don't. Take the can off. Same results. Side note, when the rounds DO light off the ejection pattern, both suppressed and non suppressed are good to go ranging form 2:00 - 4:00 ish ie nothing I am going to get hung up on chasing down.) So at this point I am thinking BCG. Take the one from my 16 inch, which I know works, and swap it out. Same results. Next I swap out the hammer spring because who knows? Nada. Same results. I had no brass case on me so left the range with a big old WTF. I mean I could understand if it was ONLY the Tula or the Barnaul but it was doing it with all three brands and out of the same ammo can that shot fine in my 16 inch gun.

Fast forward to this past weekend and after getting 100 or so rounds of PMC Bronze I go back to the range. Everything cycles, everything goes bang, runs perfectly. Everyone who I has seen the ammo (gunsmiths, ROs and dudes who are in the know) all think the primers were struck well enough that they should have gone off. So the assumption is at this point it's an ammo issue. But I keep going back to it happening with all three brands from different lots. So my question is, what did I miss in my trouble shooting? Or is this just a fluke?

Note - Failure rate on the Russian ammo was roughly 30%

Check head space

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Originally Posted by Clint

Check head space

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And actually measure the firing pin protrusion.

Can be done with a set of feeler gauges if you pay attention to where you are measuring at...

Both good suggestions.

I had checked off firing pin protrusion when I swapped out the BCG from my 16 inch (which doesn't have this issue) with the one in my 11.5 and encountered the same issue. However what I havent done is go back and see if the rounds that failed to light off in the 11.5 do the same in the 16. I can go back and measure


Any recommendations on good set of head space gauges for this caliber?

Clymer Go No Go gauges arrived yesterday. Interesting results

The 11.5 inch that is giving me problems passed both tests using two different bolts.

The 16 inch that is NOT giving me problems would not close on the GO gauge using two different bolts. Yet chambers and fires both steel and brass with no issues or apparent blow by. Bolt closes on live rounds and cycles fine. I am going to call the barrel maker tomorrow and have a discussion about this.

This leads me to believe the shorter chamber in the 16 inch is allowing deeper penetration with the firing pin. Where the in spec chamber in the 11.5 is not. The RCA BCGs do not use extended firing pins but rather machine the bolt faces to a depth where they are supposed to be not required.

I ordered an extended firing pin to try in the 11.5 to see if that helps. But right now all appearances are ammo being the cause. Now I am concerned with the 16 inch..........

Had an awesome conversation with the maker of my 16 inch barrel. When I told him about it not closing on the Go gauge he said he wasn't surprised. He said he cuts the chambers much tighter than most and while headspace is something to definitely take into account, that there are other chamber dimensions as well to take into account. And if the rifle was functioning without issue (as it is and has been) to not sweat it. Then we had an interesting conversation around bore and how, again, most folks cut X39 barrels to .308 when they should really be .311 or .312 for the X39 round. Everything he said bears truth given my experiences with his barrel. It eats everything, chambers and fires with no issues and is accurate AF. Sub MOA with Hornady Black.

So then I asked if I could pick his brain about my 11.5 inch (which is not one of his because they don't offer a barrel in that length.) Long story short he confirmed what I was thinking. Even if the bolt face is cut to the proper depth its going to need an extended firing pin if the chamber is cut deep. Pin came in on Saturday and I went to the range and sure enough it ate everything. Even the rounds that previously had not lit off and had pretty deep primer impacts.

This was a great example of how sometimes things are not always black and white. And being able to discuss with an SME sheds light on the gray areas not covered by common thought or wisdom.

He offered to cut me an 11.5 and I may take him up on that offer.

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Originally Posted by Dutch110

Even if the bolt face is cut to the proper depth its going to need an extended firing pin if the chamber is cut deep. Pin came in on Saturday and I went to the range and sure enough it ate everything. Even the rounds that previously had not lit off and had pretty deep primer impacts.

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You would have been better served in the long run if you actually tried to learn how to measure the protrusion you already had and look up what the specs were for what you were wanting that pin to light off.

You spent longer ordering a new firing pin than it would have taken you to adjust the one you had on hand already AND you paid god only knows how much extra for that 'special' pin that you still don't know what the protrusion actually measures at with your bolt.

Yeah. It was horrible. Cost me 10 bucks delivered and 3 days to get to me. Would have cost a heck of a lot more to buy a set of calipers and then what? Grind down the stock pin?