Help me figure out why my new SKS is popping primers?

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GJgo

GJgo

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Hey all,

I recently acquired a new-in-the-cosmoline Yugo SKS M59/66A1, manufacture 1982. Unissued, numbers matching. I broke it completely down to the components to give it a thourough cleaning & inspection & it all looks good. Yes, I got all the cosmo out.

Today I took it to the range to sling some lead & see how I like it. I got but a few rounds into it & stopped since they were all popping primers. Time to assess what the issue could be. The two types of ammo I had on hand were Hornady steel case 123gr SST & Federal AE 124gr FMJ. I tried rounds both with the gas tube turned on, and also with the gas tube turned off without a lot of difference in the primer popping.

The reasons I can come up with that I'd like to look into are:
- Bad headspacing
- Over-pressure *or* under-pressure rounds (for this gun)
- The firing pin is set too long
- ?

Here are some measurements. Note that all the fired cases look totally normal.
- On the Hornady cases the primer is flush with the head.
- On the Federal cases the primers are popped out .008-.015".
- On all cases the outer primer cup is pierced or darn near.

- A new Hornady round measures .441" at the web while fired cases measure .444".
- A new Federal round measures .439" at the web while fired cases measure .444".

Using a Hornady neck comparator w/ the .375 sleeve,
- A new Hornady round measures 3.218" from the base to the shoulder while fired cases measure 3.227".
- A new Federal round measures 3.221" from the base to the shoulder while fired cases measure 3.215". Huh.

- When extended fully, the firing pin protrudes .055".

I can follow up with some photos if it will help but wanted to write all this down. Any thoughts?
 
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Oh, backed out?

I thought you were blowing primers.

A backed out primer generally indicates a low pressure load without enough case stretch to reseat it after it fires.

It also indicates enough excess headspace to allow it.

But the SKS wasn't made with minimum headspace to be kind to cases, so you can reload them 20 times.
It was made to work in combat with mud in the chamber.

So unless it starts breaking cases about 1/2" - 5/8" ahead of the rim?
Don't worry about it and try some other brands of ammo that may be loaded hotter.

The pierced and leaking primers are not good though.
That can be caused by a rough, eroded, or pitted firing pin tip.

rc
 
The Federal primers were backed out, yes. Also, the primer cup was broken through in about 50% of the cases. One Hornady round even let the magic smoke come out the back of the action.

Another note, in the Federal cases the primer pockets are still nice & tight.

Looking at the firing pin tip now, it's brand new as is everything else. The tip is nice & smooth.

Looking at how massive the hammer is in the SKS, my first thought in all this was that the firing pin is set too long & the massive hammer is just driving it straight through to the anvil- but I don't know how .055" protrusion stacks up in comparison.
 
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I agree with RC - I usually don't see backed out primers unless I'm on the light end of a light load.

What diameter BULLETS are in those cases? .308? .311"? .312"?

The barrel is (very likely) .312, so a .308 won't generate as much pressure.

Got a chrono? Would be curious to know what sort of velocity those are going out with.

The Federal rounds... they SHRINK? That's unusual, to say the least. How much carbon is around the neck and shoulder? Shouldn't be MUCH. If the front of the cases are heavily fouled, I would *suspect* you're shooting .308 bullets in a .312 chambered rifle, which gives gas room to get around the NECK of the case, which would mean there's gas pressure FORWARD of the shoulder, equalizing the pressure and keeping the case from GROWING (perhaps, even shrinking it).

That's a lot of IF's and speculation though. :)

If you have the time and inclination.. slug the barrel and see what the bore size is. Then get ammo that matches. (e.g. .311 is fine for .311/312 barrels, .308 for .308...).

You can use the "wrong" bullet diameter without any problems, obviously, but could cause some weird crap.
 
Out of curiosity, did you get the SKS from Classic Arms? I got one from them about two months ago for $300 that's exactly what you described: like new condition, all serial #s matching, made in 1977 judging by the serial #, and coated in cosmoline.
 
OK here's a pic. Note in the photo I did already push the primers back in so I could get comparator measurements. You can see the carbon leak-through in the two that pierced. Also, note the roughness around the pin rim- this is a rough spot in my bolt where some material sluffed off presumably when the FP hole was drilled.

P.S. this was a Century Arms import.

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That right there is your problem!

You got a bad bolt with an oversize & chipped FP hole.

rc
 
Holy goat testies.. those are some ugly pics. Now I see why you were so concerned.

RC is spot on, there's too much gap between your firing pin and the firing pin hole the pin protrudes through. I'd also suspect that the pin is protruding around .015" too far, which doesn't help matters, as it's stretching the heck out of the primer cup.

You can fix one, but not the other.
 
rough spot in my bolt where some material sluffed off presumably when the FP hole was drilled.
Click to expand...
More likely damage from dry-firing.

With no primer to stop it, the firing pin impacts the inside of the firing pin hole and eventually causes it to chip out like yours.

Yugo's seem more prone to it then Chinese or Russian ones.
Perhaps a bolt hardening or heat treat issue to begin with?

rc
 
thats a firing pin problem. i have a norinco and a yugo 59/66a2. both pinch the primers in half he depth of those in the pics. i would get a new lower bolt. e gun parts should have them in stock.
 
Thans everyone for your help so far. Here's another photo to consider. When I realized that the tip of the firing pin is tapered I thought possibly the chipping had made the firing pin hole too big. Turns out the hole is .097", not sure how that compares to spec.

If that is the problem, perhaps I could solve the problem not by replacing the bolt but by swapping to a sprung firing pin from Murray's?

Link to photo

attachment.php
 
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Trent, both rounds use .310" bullets. From what I've also read the Yugo bore is likely .312" because they didn't get chrome lined. I didn't chrono the rounds today, but I can say the carbon on the neck is normal & looks fine.
 
For the record, you should probably avoid using Federal ammunition in any gun with a free-floating firing pin. Their primers are notoriously soft and are thus prone to slamfires and puncturing. That, and the fact that you're paying WAY too much for 7.62x39 if you buy Federal or Hornady. Buy Wolf for bulk shooting, PRVI Partizan for accuracy or hunting (they have an evil soft point). With those brands, you're getting the military-grade primers that these guns are designed for.

The new firing pin kit will probably fix your problem. And if it doesn't, it's still a good investment. Go for it.
 
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Not saying you did so, but a SKS does not take kindly to dry firing. (the same for AK and variants to a lesser extent) If there is a raised portion on the bolt face around firing pin hole, (looking like a little volcano) popped primers are almost assured.
 
Looks like a common problme. They just told you how to fix it. http://www.murraysguns.com/poppedprimers.htm
We start by fully disassembling the bolt assembly and with bolt gripped between padded jaws in a vise, we carefully stone the “Volcano” down to normal bolt face level. But, No More! A small stone, laid flat on the bolt face, can easily remove the raised area with a moderate amount of pressure and rubbing back and forth. But don’t take too much, as excessive headspace problems can arise if too much is removed from original bolt face. After that is done, we use an 82 degree countersink bit, between the thumb and forefinger, and with hand pressure only, we very slightly countersink the firing pin hole. Ten to fifteen thousandths deep is plenty. This will create a tiny raised edge around the firing pin hole, so again, lay the stone on bolt face and very lightly remove the ring that you created. That’s it.

Now there will not be a raised area that is the first part to contact the primer. The entire bolt face will, as it should, contact back of cartridge and primer. And no more sharp edge to act as a cutter and cut a hole in the primer.
Click to expand...
 
I'll try & fix the bolt photo link when I get home, but the bolt face has a crater around the FP hole- not a volcano. You can see its counterpart in the fired primers.

*edit- I can see both photos just fine at home, but at work the second photo was broken so I don't know?? I just put a link to the photo in that post too.
 
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I would try some Russian ammo before going any further. That is primarily what the guns were set up for. The primers are usually seated quite a bit deeper than American stuff.

That is why most folks have problems with their Ruger Mini 30's and light primer hits, yet the Rugers will work fine with American ammo.

If you still have problems with Wolf, Herters or Tula etc, then I would look into replacing the bolt.

I had a similar problem with an AK, and ended up replacing the bolt, and it solved the problem. In my case though, I was getting pierced primers with Russian steel cased ammo, so it was a worst case scenario. In your case, I think a change to Russian steel case ammo will take care of your problem.
 
I would try some Russian ammo before going any further. That is primarily what the guns were set up for. The primers are usually seated quite a bit deeper than American stuff.

That is why most folks have problems with their Ruger Mini 30's and light primer hits, yet the Rugers will work fine with American ammo.

If you still have problems with Wolf, Herters or Tula etc, then I would look into replacing the bolt.

I had a similar problem with an AK, and ended up replacing the bolt, and it solved the problem. In my case though, I was getting pierced primers with Russian steel cased ammo, so it was a worst case scenario. In your case, I think a change to Russian steel case ammo will take care of your problem.
Click to expand...

The SKS and AK are designed to hit the primer very hard. I agree to try steel case commie ammo first since it has a harder primer. I've found US brass ammo has softer primers; I've pierced a few.
 
You're looking at a new bolt. Once the primers started popping they leaked gas back and made the FP hole bigger.

The right fix is a new bolt.

BSW
 
After conferring with Ben Murray I'm going to try a few things to see if I can sort this out.
- Round the edges on the original firing pin tip & see if it helps.
- Install one of Murray's sprung non-tapered firing pins & try it.
- Stone / file the bolt face to make sure there are no "volcano" ridges.
- Try some Russian ammo with harder primers.
- Check headspace with a feeler gauge using a stripped bolt manually closed on a chambered round.

As it turns out, my FP hole diameter & FP tip protrusion are all within spec. The chipping on the face is just a couple thou, and it's all on the surface. Still plenty of meat in the hole.
 
Checking headspace requires a gauge. If you don't have one call around to find a local smith that does.

A cartridge and feeler gauges or tape won't tell you much.

If you lived in the PDX I could hook you up with a 7.62x39 FIELD gauge.

BSW
 
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