FED GMM Match blowout

[oldscot3]

I was shooting some groups with my Howa 308 using a box of factory Federal Gold Medal Match yesterday. After one shot I noticed a little whif of smoke come from the chamber area of my rifle. Close inspection revealed a small slot shaped hole just above the web area on one side.

I hope it didn't blemish the chamber on my rifle. I know leaking primers can etch a bolt face; what do you think, is it possible this small leak might have marred the finish inside the chamber?

[Jimro]

Yes. But it may not affect accuracy at all given the location.

Jimro

[Scorch]

One leak, probably not. Either way, it would be a minor thing and would not affect the rifle's performance.

[oldscot3]

It seemed pretty minor; the only indication that something out of the ordinary happened was the small wisp of smoke that I mentioned. I suppose that piece of brass had a contaminant embedded there(?), I'm not sure. I've seen them blow through a wrinkle in the neck but that's the first factory load I've seen fail like that on its first firing.

I'll take it out of the stock and give it a good cleaning, maybe I can see in the chamber a bit with a good light. I'm not expecting damage really unless it's perhaps a minor blemish. Regardless I'm a bit annoyed; OCD thing I guess.

[Mobuck]

When seen on a consistent basis, this often indicates an excess headspace condition. In a single case instance, it was more likely a defective cartridge.

One of my older .308 Mauser action target rifles had headspace in the "longer than safe" range meaning the case head blow-out was consistent. It was fine with tailored handloads and was as accurate as most rifles but would blow out 1 of 3 factory loads. I decided to be safe and had it fixed before someone besides me loaded it up with factory loads and got hurt.

[oldscot3]

I've had some surplus rifles with max headspace before. Brass cases coming out appeared stretched out all the way around the case, usually looking like they had a shiny ring right where the base web intersects with the case wall. Continued firing would cause a separation (and massive pressure leak) along that ring. This little leak is slot shaped and oriented end-to-end.

My new Howa closes on a factory new round with a slight amount of "feel". While I haven't put headspace guages in it, I suspect it does not have excess space, rather I think it has a pretty tight (minimum spec) chamber. Another thing that makes me think that, is that when I resized some cases that were once fired in another rifle, I had to adjust my sizing die a bit. Those cases were closing with a lot of feel.

[4EVERM-14]

That would be something the manufacturer would like to know about. The case apparently did not form correctly. Send them the box serial number and a picture of the case.

[FrankenMauser]

Looks like a defect in the brass, to me.

The chamber was certainly etched by the failure, but it shouldn't be enough to cause a problem. You may not even be able to see it with the naked eye.

[Slamfire]

Brass flaws happen, could pit the chamber, but maybe not. You just have to look. Another thing that has caused problems such as you experienced is old ammunition. Old gunpowder outgasses nitric acid gas and that attacks brass. You will most likely experience a lot of cracked case necks before you have pin hole erosion through the sidewall, but I have experienced both with old ammunition.

These are old 8mm cases, one has pin hole erosion high on the body, probably did nothing, but the other case crack, right through the case head, that is very bad as gas goes right back into the action.



Cartridge cases are not structural members, they are not supposed to carry load. They are like gaskets, they have to be supported of they will blow.

One of my older .308 Mauser action target rifles had headspace in the "longer than safe" range meaning the case head blow-out was consistent. It was fine with tailored handloads and was as accurate as most rifles but would blow out 1 of 3 factory loads. I decided to be safe and had it fixed before someone besides me loaded it up with factory loads and got hurt.

I am going to be nit picky and say the problem you had was really not "head space" but cartridge case head protrusion. They are related because the assumption is that weapons that headspace correctly, are properly built and the case head protrusion is correct.

These pictures came from Chinn's series on the Machine Gun. He is showing the effect of case head protrusion and case movement during extraction.







Automatic and semi automatic mechanisms have to be carefully timed. Cartridges in all of these mechanisms are moving before residual barrel pressures are zero. Therefore how much of the case sidewall is out of the chamber is very important because if residual pressures are too high, the sidwall will rupture. As the pictures show, case sidewalls thin out quickly.

In bolt rifles, the Mauser bolt rifle was designed to provide more case head support than a 03, for example.

Mauser



M1903



The Mauser design does an excellent job of seating the case as much as possible within the chamber, very little of the case head is exposed. Some has to be, because you want the extractor to pull the case out.

I expect that whoever built your Mauser removed material that should not have been removed. There could have been a deep bevel on the barrel breech, or the gunsmith could have lapped the lugs, cut the receiver seats, or removed material from the bolt face. Any of these actions would have the effect of exposing cartridge case head out of the chamber, even though the headspace was correct. Headspace is a measurement of base to shoulder, but, the assumption with all weapons maintenance, is that someone did not monkey with the case head protrusion.

You can google 40 Caliber Glock case head kaboom and find lots of pictures of blown 40 S&W cases in early Glock 40 pistols. https://www.google.com/search?q=gloc...ih=700&dpr=0.9 These early Glocks allowed too much case head to stick out on the ramps.


This action, used in the Oerlikon, provided very good case head support. It required greased cases to function. By the time the case was ejected, breech pressures were virtually zero, but, as the upper picture shows, prior to that point, barrel pressures were high enough to rupture the sidewalls. Pre greasing cases and linking them up was messy to after WW2, oilers were installed on the weapons.



Most shooters are totally ignorant of these weapons, primarily because they don't read, and that few people can afford to own a 20mm machine cannon. But the things did exist, and did require greased or oiled ammunition to function.