Baffled: pierced primers. Please look over my shoulder.

[doofus47]

Ok, I'm often slow to actually sit down and begin to solve problems. Generally, though, once engaged, I can suss out the cause (if not the resolution) to a problem. This issue has me buffaloed.

Problem: my MGI 7.62x39 "enhanced" Bolt and firing pin sometimes pierces primers on steel-cased ammo.

By comparison, the stock bolt/firing pin combo that came with my Bushmaster 7.62x39 upper doesn't pierce primers, but it fails to ignite a small fraction of steel-cased ammo even though it dings the primer mightily.

Assumption 1: The MGI firing pin is too long.
Actuality: I put both firing pins side x side and assumption 1 was wrong. I measured with a caliper and confirmed that the opposite is true. The stock firing pin is actually .005 longer.

Assumption 2: The firing pin channel on the bolt of the MGI is milled differently such that the shorter pin protrudes farther than stock firing pin b/c the interior has a deeper channel.
Actuality: When the firing pin is fully extended through the bolt, the stock Bushmaster is still a bit longer than the MGI. I didn't measure, but there was a visible difference.

Assumption 3: the MGI firing pin is sharper than the stock Bushmaster firing pin.
Actuality: No, the Bushmaster is a more narrow by a tiny bit.

Question: Why and how could the MGI be piercing primers while the stock Bushmaster firing pin not be doing so? Judging by the measurements I've taken, the Bushmaster firing pin should be piercing primers.


This is not a life or death issue; the MGI bolt/firing pin is a backup, but I'm a fan of consistency. Where else can I look?

[Mobuck]

Is there any weight difference?

The other so far untested diff might be actual headspace. If headspace with the aftermarket bolt is tighter, the FP would punch deeper into the primers.

[Jimro]

Wolf/Tula brand ammo has been known to pierce primers in SKS and AK platforms. It's cheap for a reason, so unless you are using some other steel cased ammo, I'd say that's probably the biggest source of your issues right there.

If the Bushmaster bolt and pin aren't piercing primers, then I agree with Mobuck the "headspace distance" between the bolt face and forward chamber shoulder (where the cartridge stops moving forward) must be greater than the distance with the MGI "enhanced" bolt. At least until more information is provided that is the most logical explanation of the information given.

If you really want to fix the issue, you'd have to stone the firing pin ever so slightly back until the issue goes away.

Jimro

[doofus47]

Thanks for the headspace suggestion.

I didn't think about that at all.

[44 AMP]

Does the problem occur with quality brass cased commercial ammunition??

Or, is it only with certain steel cased ammo??

and if it is, is it REALLY a problem??

the stock bolt/firing pin combo that came with my Bushmaster 7.62x39 upper doesn't pierce primers, but it fails to ignite a small fraction of steel-cased ammo even though it dings the primer mightily.

Have you tried (and do they go off??) a second strike with the stock parts?
Have you tried firing the "dud" rounds with the aftermarket parts? DO they go off??

My assumption #1
the ammo being used is cheap crap, clearly not optimal for your weapon. (if it was, you wouldn't have EITHER issue)

Assumption 2
you are looking for something wrong with your rifle (and its different parts) before (and without??) considering the problem may be the ammunition, not the rifle.

Try other brands of ammo. IF the problem persists, THEN you look at the rifle.

[doofus47]

Does the problem occur with quality brass cased commercial ammunition??
D47: I have about 40 rounds of brass 7.62x39 that i keep for hunting. That ammo has never failed. My 7.62x39 AR is for plinking more than anything, so steel is the most commonly used ammo. My problem is a steel issue, not a universal issue.

Or, is it only with certain steel cased ammo??
D47: Good question, I have yet to go through my crap load of Tula to find out.

and if it is, is it REALLY a problem??
D47: This is not the worst problem anyone will look at on this forum. The cartridges all go 'bang.' I'd like to avoid the black smoke and accumulating brass shrapnel in my BCG and lower.

Quote:
the stock bolt/firing pin combo that came with my Bushmaster 7.62x39 upper doesn't pierce primers, but it fails to ignite a small fraction of steel-cased ammo even though it dings the primer mightily.
Have you tried (and do they go off??) a second strike with the stock parts?
Have you tried firing the "dud" rounds with the aftermarket parts? DO they go off??
D47: they always go off. I used to re-load the non-firing rounds into my SKS and they would always fire.

My assumption #1
the ammo being used is cheap crap, clearly not optimal for your weapon. (if it was, you wouldn't have EITHER issue)
D47: agreed; I'll check this.

Assumption 2
you are looking for something wrong with your rifle (and its different parts) before (and without??) considering the problem may be the ammunition, not the rifle.
Try other brands of ammo. IF the problem persists, THEN you look at the rifle.
D47: Fair enough. I have some Uly steel-cased in a box somewhere. I'll compare using that.

Thanks for the input.