Prevention of Slam Fires

victorj19

I want to load the 7.5 Mas for a nephew that has the bolt action and semi auto rifles from WW II. I searched a bit on this forum and others and learned about the slam fire issues with the semi auto when using standard commercial primers and military primers, mostly CCI, were recommended.

Would the TULA brand primers also work? I seem to recall that TULA primers use thicker metal.

noylj

Not being an expert, but my thoughts:
You could always place an empty primed case in the chamber and close the bolt (at the range). When removed, see if there is a mark on the primer. That would indicate if there is a potential problem. Then, if there is a mark, I would take the gun into a gunsmith and it looked at.
You also might want not to use the cheapest primers you can find but buy the primers made for military rifles--maybe CCI #34 or #41 primers.

Nordicwargod

A friend has a MAS 49. He doesn't reload for it and the only ammo he can find is Syrian produced 7.5MAS made back in the early 50s at gun shows. That being said he does not have this problem so it could be soft primers if it has a floating firing pin. Doing the empty case with a live primer and letting the bolt "slam" forward may let you know if this is your problem. Do it with one you primed and pull the bullet on a military round and compare.
From what I remember Century Arms imported most of the MAS rifles, inspected them, and re-barreled many to shoot .308. I have read many complaints of their quality control of any work they did with these rifles. Having a good gunsmith look at it may be a good idea.

jonk

I had that very problem. After reading about expensive fancy titanium firing pins, I just took out the existing pin and filed a tiny bit off the tip, making the pin flat rather than rounded. Just a touch mind you. That ended the issue.

However, not ALL MAS rifles do this, so I'd say you have to try it before you know for sure if it is an issue.

Emmett Dunham

I would say that if the rifle is in good condition and it is an inherent part of the rifle design that the primer is dented when the bolt closes and you do your job reloading the ammo there should be no problem. I have three rifles that dent the primer every time the bolt closes on a round. The SKS really marks the primer up and I have never had a misfire. Find out what the specification for firing pin protrusion is and measure this on the rifle you are going to reload for. Do some test loads and take the rifle out and fire it and see what you got.

jonk

quote:Originally posted by Emmett Dunham
I would say that if the rifle is in good condition and it is an inherent part of the rifle design that the primer is dented when the bolt closes and you do your job reloading the ammo there should be no problem. I have three rifles that dent the primer every time the bolt closes on a round. The SKS really marks the primer up and I have never had a misfire. Find out what the specification for firing pin protrusion is and measure this on the rifle you are going to reload for. Do some test loads and take the rifle out and fire it and see what you got.


That's a nice theory, and in a perfect world you would be right. However, you have to factor in that original milspec ammo had a rebated primer that was harder than most commercial or even CCI 34 primers. All that combined means even if everything is in spec, you get slam fires with many MAS rifles.

Mobuck

I reload for a bunch of Mausers(both original and sporterized)and have had some issues with primer perforation. On some of these rifles, I've shortened/rounded the firing pin and solved the problems. Some got the same treatment but I still use CCI#34 primers since I use powders that like magnum primers anyway. I use CCI#41 in nearly all AR reloads. This primer increases group size slightly and requires a slight reduction in powder charge.
If you're only loading ammo for milsurp rifles in a certain caliber, I don't see it being a great hardship to use the milspec primers.

best defense

One of the main reasons for slam fires is a fireing pin that extends from the face of the bolt when the bolt is closing.

As someone else suggested, check to make sure the firing pin does not extend from the face of the bolt or touch the primer unless the trigger is pulled.

There are a couple of reasons why that might happen, but only a gunsmith should try to fix it.

v35

Depending on the design and your abilities, you can reduce the weight of the firing pin by removing metal or by installing a return spring. A titanium firing pin would be the easy way if they were available.
Blunting the firing pin tip slightly might work.
It's not a question of protrusion but of inertia and momentum of a heavy firing pin. Protrusion with the firing pin rear against the rear of the breechblock should be about .060"extending from the front of the bolt.
Piercing is a different issue from slam firing.