Semi Bren Primer Flow

[TactAdv]

You have more problems with that ammo than you know. In each cartridge case shown the base metal of the case head is decomposed/eroded on the interior rim at the primer pocket opening. The non-uniform surface deformation/ inconsistency visible appears to be the result of corrosion or mechanical abrasion.

Are those reloads?? That ammo would fail any ISO verification check given at original manufacture. If you bought them loose in a brown cardboard box from ATK/Federal, they are "factory seconds", failed acceptance (scrap), that's what ALL the Federal labeled bulk "MILSPEC" is being sold commercially.

I'd start with some other ammo before you go on a wild goose chase with your gun.
-TomH

[cipherk98]

First time shooting this gun, first time shooting this gun with that ammo, or neither? If it works fine with other ammo and only does it with that stuff I'd bet on the ammo.

Also, the bulk Federal (also white box) is crap in my experience. I've had some with super weird tarnish (orange spots being the most memorable) or damage. Would never reload any of them.

[ActionYobbo]

Had the same problem with my 303 bren. firing pin is moving too far rearward. the firing pin should not be able to go lower than the bolt face. fix that issue and you wont have that problem.

for Peter
with the semi conversion the firing pin is free to move after being struck by the striker. The cross pin on the breach block is what is used to control firing pin movement forward and rearward.

[browningautorifle]

Are you putting TNT in your reloads?

Those are factory ball ammo Peter... Unless someone has pulled the bullets and changed the loads and reseated bullets...which I've seen done...

LC 16, last years ammo. I don't think there's corrosion at the primer, I think that's from firing.

[alterego]

Peter, please look at the corners of the primer, they don't look flattened...at least to me. Otherwise, yes it looks a bit ugly.

Please bear in mind that a semi is a bit different than the full auto gun as well. My firing protrusion looks correct (was same as standard pin too). However the firing pin (aft end) was about 2mm too long. So in a full auto gun the piston post whacks the firing pin, and gets it moving at whatever velocity that impulse gives it, then impacts the primer pin at whatever momentum the firing retains at primer impact. (I know you know this, just spelling it out in case _I_ have gotten something wrong)

In the semi gun, the bolt closes, the moving mass is the striker, telescoping section of the recoil spring and half the recoil spring's mass. Bear in mind that the striker is propelled by say 1/3 the total of the recoil spring force. So I think it's safe to say that it is at least a possibility that the semi striker is imparting a greater impulse to the firing pin than a full auto gun.

Now, all that being said, the aft end of my firing pin was ~2mm too long (sigh). So rather than hitting the primer with just the momentum of the firing pin's mass x its velocity, the pin is hitting it with whatever velocity remains x the mass of the striker, telescoping recoil section, half the spring AND the firing pin.

I've shortened the aft end of the pin for about 0.35mm free play, hopefully I don't get light strikes. I'll shoot it and take photos again.

As for other questions I might have missed, the ammo is new and known to be a good load (NATO M80 I think).
Headspace is where I set it at build about 0.003" loose from go (can be forced closed on no-go gauge), chamber is good condition (chromed).
The firing pin is a modified full auto pin (tail added to pin).

[browningautorifle]

As for other questions I might have missed, the ammo is new and known to be a good load (NATO M80 I think).

That's what it appears to be too...

[Peter Laidler]

Getting back to the mechanical basics chaps........ SOMETHING has driven the primers rearwards with such force that the centre sections have been impressed deep into the striker hole in the breech block. As I sit pondering this, harking back to my days in the bench, there's only two things that can cause this. The first is excess pressure inside the cartridge case attempting to take the line of least resistance. And the second is a full harvest mood upsetting the dynamics of mankind with a magic influence. Take your choice. But one thing you can be sure of. It ain't excess FPP. That is coincidental to the blown primer

[browningautorifle]

I had occasion to shoot Hurtenburger Patrone about 25 years ago and even though factory ammo and top rated match ammo I had signs like that. New ammo out of the box...try different ammo.

[alterego]

LOL, Peter, I tend to agree with you...but I cannot find anything in the gun that would drive an overpressure situation. If you have any advice I'll work in those areas.

I'll try some more of this load (in other guns too), get some other flavors of ammo, and grab some pics.

[ActionYobbo]

I posted the cause and cure for your problem. Untill you cure the firing pin travel you will have this problem. The primer can not form into a recess the is not there. The amount of primer protrusion is equal to the amount your firing pin is below the bolt face. The firing pin is not out the whole time when firing like a bren or mauser it hits and bounces. The primer is blowing back into the unsupported area that is the firing pin hole, it goes in as far as the firing pin retracts.

Or you can use winchester primers and loads equivelent to 30-30