Primer blow-outs in 7.62 x 39 mm & Mini-30

[Bill_Rights]

Question on cartridge design/function: (please forgive this slight diversion from the facts of the case.)
Is a rifle primer supposed to be so tightly seated in the base of the brass that the primer will remain in place during the explosion of the powder, even if the primer is NOT supported by the bolt face?

I ask this because I had some primer blow-out events while shooting recently. The rifle was a Ruger Mini-30 Catalog Number: M-30/20GBCPC | Model Number: 5854 | Caliber: 7.62 x 39mm, which is the police/tactical model with barrel length 16.12".
The ammunition was Prvi Partisan 7.62 x 39mm 123 gr. RNSP (round-nose soft-point) with brass cases and boxer primers. Here are photos of some cases with blown and not-blown primers, plus a couple of unfired cartridges. I would say that about 1 out of every 4-5 shots blew a primer.



When a primer would blow, the semi-auto action cycled OK, except a few times a loose blown primer prevented the bolt from closing on the next round.

This is PPU brass, and the origin of the primer is unknown/unmarked (to me). On the headstamp you can see "ППУ" which is Serbian Cyrillic for "PPU".

http://en.wikipedia.org/wiki/Serbian_Cyrillic_alphabet
Pe (П п) Cyrillic letter Pe was derived from the from the Greek letter Pi (Π π) and related to Latin letter P
U (У у) is a letter of the Cyrillic script, related to Greek letter Upsilon and Latin letter U
So this head stamp means "PPU" in Latin/English, which is the acronym for PRVI PARTIZAN UZICE Serbia

MY QUESTIONS:

1) Is this just brass with mis-manufactured primer cup or just mis-manufactured primers that don't seat tightly enough?

2) Could it be that the semi-auto action is not closing fully, so the bolt face is not backing the cartridge base when the powder goes off?

3) Could it be that the semi-auto action is starting the eject stroke too soon, so the bolt face is withdrawing and not backing the cartridge base while the chamber pressure is still too high?

In possible support of choice #2, the rifle is fairly new, only about 100 rounds through it; so maybe the action is sticky and the bolt not seating fully every time?
In possible support of choice #3, this particular ammo has the sharpest initial recoil (initial jerk) of any 7.62 x 39mm 123 gr. I have shot, both in this rifle and the same ammo compared to other ammo in other rifles. I think this loading has a fast-burning powder (which should be good for a 16" barrel, right?). But by the same token, the gas-driven cycling of the semi-auto action might have too high a pressure too soon, thereby starting the bolt withdrawal and ejection too soon.

What're your thoughts?

 

[wally]

Is a rifle primer supposed to be so tightly seated in the base of the brass that the primer will remain in place during the explosion of the powder, even if the primer is NOT supported by the bolt face?

No way, but if the primer is not supported by the bolt face at ignition something is really wrong! The primer always backs out a little, but then gets reseated when the full charge ignites.

Since your brass is not split or bulged (apparently) I'd suspect #3 is the root of the problem, aided by #1.

#2 generally results in a KaBOOM! as only a very small amount of unsupported brass can withstand the peak pressure.

I've reloaded a lot of brass a lot of times and have never picked up a just fired empty without an expended primer in the case, but it happens and that's why military ammo generally has crimped in primers.

 

[Bill_Rights]

Thanks wally.

If #3 is the root problem, and I tentatively agree, is it possible that
A) the rifle action just needs more rounds and cycling to break in, then the problem will go away OR
B) I need to figure out how to turn down the gas tap or reduce orifice size on the gas that cycles the action?

 

[M1key]

Try some other brass-cased ammo and see if it still does it, like Winchester, PMC, or Fiocchi...if not it's probably just the ammo. I don't see any signs of excess pressure on the primers or cases so you're okay there.



M

 

[Jackal]

First thing I would do is try different ammo. Without testing other ammunition, its difficult to say whether the problem is with the rifle or the ammo. Try another brand and check back. If the problem persists, you have a rifle issue. If different ammo cures it, then PPU is to blame.

 

[wally]

Try another brand and check back. If the problem persists, you have a rifle issue. If different ammo cures it, then PPU is to blame.

I wouldn't quite go that far, I think the rifle has to share some of the blame for opening with enough pressure to blow out a factory primer.

But in any case, if it does it with another brand of ammo I'd say sending it back to Ruger is your safest bet!

 

[Bill_Rights]

M1key & Jackal,
Yeh, I think we did try some PMC "Bronze" FMJ and some Federal "American Eagle" MFJ. They worked fine. But, I am not totally sure we tried these brands in this exact rifle. Our family unit got three Mini-30s all together at Christmas 2011, and we were running all sorts of ammo through each of them. The GBCPC was the last one and least used in those days, so I can't exactly recall how much, if any, of these other ammos we ran through this rifle. I guess it's back to the range.

I am not the only one who has noticed the sudden/early recoil of this particular PPU loading. Can't remember where I read it. Other people on this board or maybe over on Perfect Union Mini-14/30 forum.... There're a lot of 7.62x39 shooters over there on Perfect Union Mini-30. I am pretty sure all the brass-cased FMJ factory ammo I've shot (I have not shot PPU FMJ) has a more mild onset of recoil feel.

So early release of the action lock-up due to the fast-burning powder is a strong possible candidate root cause.

wally,
Good advice - THX.

 

[Edarnold]

From the appearance of the fired cases and what the photo shows of the dropped primers, I'd suggest you have run into some very soft cases. Quick way to check for this is to get some new unfired primers and see if you can push them into the primer pockets with finger pressure. If you can, the problem is due to the case head expending too much on firing. Since the pressures appear normal according to the other fired cases, the problem must be with the metal in the fired cases.

You can also try running the fired cases from this rifle into the other Mini-30's you have, if the headspace in the problem gun is so excessive as to cause dropped primers, the cases won't go anywhere near chambering in the other guns.

I've seen dropped primers tie up ARs during rifle matches, the problem always involved maximum level handloads going through a rifle that was very hot from a rapid fire stage: in other words, way too high pressures. That doesn't seem to be your problem here. I'd spend some time evaluating the ammunition before you blame the rifle, with the push to supply ammunition during the recent shortages all kinds of ammo defects have been showing up in all brands due to speeded up production.

 

[SilentScream]

I would concur with what pretty much everyone else said. You need to run some different ammo first, without that you're more than likely spend hours/days chasing your tail trying to find an issue with the gun that may not exist.