FC 7.62x39 popped primer


#1

My rifle range pick-up (I myself cannot afford anything but bulk Russian-made). What made primers pop out? Just poor workmanship or something else? Was that gun’s fault?


#2

Low Pressure + Excess Headspace + Rough / sticky chamber walls.

Low pressure is the most common cause, followed by HS.

Remember commercial ammo doesn’t have the primers crimped in, the crimping will slow/eliminate this condition.


#3

Vlad

From the looks of the case on the left, I’d say that the case head is about to seperate which means that the headspace is excessive. Whether it’s the fault of the rifle or the cartridges themselves is hard to say.

It could also be any of the other causes mentioned by Tailgunner.

Do they look like handloads?

Ray


#4

Ray,
I don’t see any re-loading marks, but then I don’t re-load so maybe I am not looking in the right place. Excuse my ignorance, but could that be excessive “high pressure” due to a bit too much gun powder that pushes primers out? Please, explain.


#5

Vlad

Excessive headspace means that either the chamber headspace is too long or the cartridge headspace is too short. When a cartridge is fired under either condition the case walls will expand and grip the chamber but the solid head (which does not expand) will be allowed to move to the rear, stretching the case. This can usually be seen by a shiney ring or even a partial head separation about 1/4" or so above the rim. A partial head seperation can also be accompanied by a primer that is partially backed out of it’s pocket. That’s what the case on the left appears to have but I’m only judging by your photograph.

If there is gross excessive headspace, the case head will completely seperate letting gasses escape into the rifle’s mechanism. This often results in damage to the rifle and/or shooter.

A low pressure cartridge will be driven forward by the firing pin’s momentum and the primer will be driven, by it’s own pressure, to the rear. This can give the appearance of excessive headspace but it’s not.

Excessive pressure from an overload will generally flatten the case head and primer.

I’m sure you’re totally confused by now. It takes an experienced shooter to recognize exactly what the appearance of fired primers means. Maybe Taigunner or John Moss can put it into better words than this ignorant farm boy (me).

I’ll dig out a case or two that has a partial head separation and photograph them for you.

Ray


#6

Ray - thanks for the confidence. However, I can’t beat that explanation. It is the clearest put, best explanation of what causes a potential or actual case-head separation that I have read. I learned something about the mechnics of it, although I understood the phenomena.

John Moss


#7

Vlad

John is too modest.

Anyway, here’s a photo of a partial head separation. It’s darn near a full separation. The shooter may or may not have gotten a face full of hot gas. I don’t know who it was.

As you can see, it can happen with rimmed cartridges too although it’s much more common in rimless.

Ray


#8

Ray - is that a .30-30 case? I know that with .303 British, because the chambers are made oversize to overcome the weak initial extraction of the Lee-Enfield Rifles, they shed case heads after just a few loadings. Thinking of my two .30-30s, which I almost never shoot and therefore don’t bother to load for, they are jerking the cases right out of the chamber, so they have poor initial extraction also. I would guess that the chambers are somewhat oversize, another condition besides headspace that causes the same chain of events you so perfectly described.

John M.


#9

John

That particular case is a 348 Winchester. Somewhere in my collection I have a 30-30 case that looks just like it. Unfortunately it was my grandson who was shooting the 30-30 when the seperation occured. Fortunately he wasn’t hurt. He got just a small flash of hot gas to the face. He was wearing shooting glasses, of course, because that is one safety rule that I have drilled into his head from day one.

Most lever action rifles are prone to this sort of thing. They are not too tight to begin with and after repeated firing the headspace only gets larger.

I don’t know if you remember, but I think it was the Browning MGs that seemed to always shed case heads. But it never seemd to hurt them. Just dig out the remainder of the case and they started shooting again with no problems. I recall that we used to headspace them on the “long” side to be sure they would function irregardless of dirt, grease, or grime in the chamber.

Ray


#10

Ray,

Its been 50 years since I messed with the Browning 1919A4 and M1919A6 as an assistant MGer in the Army. As I recall, the quick, field expedient headspacing if you had to change barrels was to screw it in all the way and back it off two clicks. I saw a gun, improperly headspaced, blow from a blank (the gun had the standard blank-fire adaptor at muzzle and feed tray) from improperly headspacing it. It actually buckled the top cover, which is a massive piece of steel, into a shallow upside-down “V” shape and did some minor damage inside the gun as well. Incredible. I would have said that it would be impossible to bend the top cover of a Browning short of running an armored personnel carrier over it, or something. Glad it wasn’t in my squad.

Aside from this, which did a lot more to the case than just separate the head, we never had a case-head separation with blanks or live ammo of ball, armor-piercing, tracer or incendiary - all we ever fired. Frankly, we never had many problems at all with the Browning .30. Lots of people like to compare it badly to the German WWII guns, but it was more rugged than either an MG-34 or MG-42 (the only time I ever got to shoot an MG-34 it broke during firing. Don’t know what broke, but it jammed and once the jam was cleared, it was through for the day. Not our gun, so never found out what went wrong) and had a much more sensible rate of fire. The A6 version had barrel change thru the front, although not as good as the German types for that particular feature, or even as good as a Bren gun (Best auto rifle in the world!). The A4 version of the BMG had to be detail-stripped with the barrel and barrel extension coming out the back, and then unscrewing the barrel to change barrels. Very, very slow. Can’t imagine ever doing it under fire. Would probably have been better to abandon the gun and go to our personal weapons had we ever been in that situation. I was never in combat, so didn’t have to suffer through such an experience.
It was hard to change barrels on the A6 - we trained to do that - but it was touchy, as I never saw one of the abestos gloves we all were supposed to have in our gun crews. You change barrels because they are too hot!


#11

John

I trust your memory a lot more than I do my own. I never shot the Brownings very much and maybe I just remember the case head seperations more than the routine stuff. It’s also possible that the “Navy” headspace setting was a little looser than the Army. I can’t remember how many clicks we came out but maybe it was more than it should have been. Or maybe they issued all of the reject ammo to the Navy. :) :)

Ray


#12

From my experience, I would relate the backed-out primers to the condition of the chamber they were fired in. IF the chamber is really rough or dirty, when the brass case expands and obturates in the chamber, it might stick really tight to the chamber walls not expand rearwards enough to contact the bolt face. Thus, the primers move out a little until stopped by the bolt face. The second cause could be that the cartridges were way underpowered. This again would cause the case to not stretch back enough to contact the bolt face. Since these appear to be factory loads judging by the red primer seal, I doubt they were that underpowered. I have seen the dirty chamber phenomenon when steel cased ammunition is fired first and brass cased ammunition is fired right after it. In the case of .223 rem. fired in an AR-15, the steel cases do not expand as much as brass and some powder gasses leak past. This gets the chamber pretty dirty after a number of rounds. When you fire brass cased ammunition next, the brass expands more and sticks to the chamber walls and all the powder residue. This can and has casued extraction problems in some rifles. The first 5.45x39mm fired cases I obtained, way back in the 1980’s all had the primers backed out a little. Enough so that they would not sit flat on my display shelf. I suspect that these were fired in an AK-74 either in Afghanistan or at a U.S. Army facility using a captured rifle. In either case, I doubt that the rifle was very clean at the time of the firing.

AKMS


#13

Now I understand the concept of “low pressure”. When the pressure inside the cartridge is less than that required to dislodge the crimped bullet, that pressure goes backwards and pops the non-crimped primer. Sincere thanks to everyone for enforcing “no-old-man-without-knowledge-of-ammo-left-behind” policy.


#14

Vlad - you are still not entirely clear on the low pressure situation. It is not that there is not enough pressure to push the bullet out of the case - even a primer alone will often due that, depending on the caliber, and the amount of bullet crimp. It is that the low pressure of the cartridge does not allow enough case set-back for the head of the cartridge to be firmly against the face of the breech (bolt, slide, etc.) This gap allows any residual pressure pressure in the case to move the primer out of its pocket until it contacts the breech face and stops. One other sign of low chamber pressure is the failure of the case to obturate the gasses, in which case the outer case walls will be blackened. Some mistake this as a sign of high pressure. It is normally the opposite, as there was not enough pressure inside the case to have it perform one of its normal functions, and that is to expand tightly against the walls of the chamber as a breech seal.

If you have a revolver, and want to see how this works, take out the bullet and powder and fire only the primer. The primer will normall back out and lock up the action to the point where you cannot rotate the cylinder by hand or by cycling the action. This is then easily fixed by putting a wood dowel down the barrel, into the cylinder and empty case, and then tapping the end of the dowel with a mallet to push the case back down over the primer.

I hope that I have explained this well and clearly. I have what I feel is a pretty good grasp of firearm dynamics, but I have a helluva time explaining them sometimes. Others on this Forum do a much better job of it than do I.


#15

A classic example of this failure to obturate and allowing the gasses to escape past the case is the time I down-loaded some 7.62x54r for my kids to shoot. I handloaded some cartridges with about 20% less powder to tame the recoil a bit. It worked, but the gasses blowing past the not fully obturated cases was not pleasant.

I did not look to see if the primers were set back any, as I was busy cleaning the crud off of my shooting glasses! I’ll have to go check the scrap brass bucket to see if any had this happen.

AKMS


#16

It’s incredible what low pressure cartridges sometimes do. I have examples in my collection of case necks that have been collapsed and some even mangled making them useless. Splits in the shoulder that started on the outside and blew into the interior of the case. And there is that rare, but very real phenomena called “Secondary Explosion Effect” (SEE) that has actually destroyed rifles and injured shooters.

Shooting is still safer than driving on the Interstate but never dull or uninteresting.

Ray


#17

OK, Ray, you got me hooked. What is Secondary Explosion Effect?


#18

Vlad

I was afraid somebody was going to ask that.

“Secondary Explosion” is really a misnomer because it hasn’t been proven that there are two seperate powder “explosions” or detonations. It’s a phenomenon that is not easy to explain mostly because no one is absolutely certain why it happens and efforts to duplicate it under laboratory conditions are rarely successful.

In a nutshell, when a very large cartridge case is loaded with a small amount of slow burning powder, the result is usually very low pressure and low bullet velocity. But, every once in a while the rifle blows up.

One explanation is that the primer flash will ignite part of the powder charge creating just enough pressure to push the bullet partly into the bore where it stops. Then, when the ignition spreads to the remainder of the powder charge the bullet cannot accelerate again fast enough to keep a dangerous pressure from rising. Kind of like a bore obstruction.

Another idea is that because the powder charge is so small in such a big case that the primer flash is able to ignite the entire charge all at once, rather than progressively, making the slow powder act like a very fast one.

Another theory is that the small amount of powder is blown forward in the case, compacting in the neck area, firmly against the base of the bullet to the point where, again, the barrel is plugged exactly like if another bullet had been left in the chamber’s throat.

You get the idea. The simple fact is that it DOES happen but, as I said, it is not predictable or easily duplicated.

Fortunately it can be avoided by simply using the right powder in the correct weight when loading the very big magnum cases. Unfortunately, not all handloaders believe that it really happens and still try to load mild loads in big cases. Every once in a while one of them gets an abrupt lesson.

Now my head is hurting and I’m going to bed.

Ray