Semi Bren Primer Flow

[Peter Laidler]

Nope......., I most respectfully slightly disagree with your mechanics MrE but agree with the ammo observations. I don't happen to have a Bren BB with striker here and fully understand the geometry regarding the depth of the striker below the breech face. BUT, you're forgetting that it really doesn't matter how deep the striker is set back but how much it protrudes. If it did matter how much it was permitted to be set back, there would be a gauge for it. The depth of set back or return into the BB is all to do with the length of the recess for the transverse striker retaining pin in the striker - about 9mm or so from memory. As I said, I don't really understand the mechanics of the US re-welds so I could be talking pure horse shi..........er....., manure. In the meantime, it can ONLY be gas pressure or magic that is blowing those primers.

You could try different ammo to eliminate that OR a new striker OR a new BB.

[ActionYobbo]

Peter you are trying to reinvent the wheel here. This problem has popped up before in striker fired brens its the firing pin.
If i am wrong I will buy a round of beer for everyone who has posted above here

I know where you are coming from Peter but the semi auto bren firing pin has more in common with the SKS than it has with the original BREN. Take an SKS with an eroded firing pin tip and lengthen the slot for the retaining pin so you get FPP and you will also get pregnant primers.
In this case the slot in the firing pin is cut too far forward allowing excessive rearward movement of the firing pin. You have 12oz of firing pin
and striker against 2 ton of pressure on the primer cup. Pure physics at work here

To Mr alterego can you post a picture of your bolt face with the firing pin retracted to its rear most travel. I assume you have a spring in the front of the firing pin to hold it back and by your description I also assume you are using mightylime's setup

[Vincent]

In this case the slot in the firing pin is cut too far forward allowing excessive rearward movement of the firing pin.

Yes. That’s the problem.

Maybe a picture of a semi-auto firing pin will help everyone visualize what’s happening.



The red arrow marks the area Mr E is talking about.

BTW. The firing pin protrusion is set by the back of the slot in the pin.

[tr63]

The firing pin shown looks like the one in my Wise-Lite rebuilt semi-auto Bren in .303 . None of the fired cases from my Bren show primer flow back . The wise-Lite firing pin is still made by a company in Boynton Beach, FL.

[Vincent]

Wise Lite Arms still sells the firing pin and other parts http://www.wlarms.com/default.asp

[alterego]

Yes, it's Mighty Lime's (an individual who relishes using damned Bren guns to hurt my feelings) most excellent setup. Will try the firing pin ideas. Will attach image of same ammo shot through an FAL for a reference point...Peter I'm assuming you're more than familiar with those (as L1A1's).

Firing pin is 0.010" below the BB face, original firing pin return spring is in place.

[browningautorifle]

Peter I'm assuming you're more than familiar with those (as L1A1's).

This ought to be good...

[Peter Laidler]

I'm sort of fairly familiar with L1A1's, in a bit part amateur sort of way. Much the same as Brens really...... And those cases (thread 26) say it all....... Maybe you should fire the same ammo in an Indian 7.62mm 2A rifle where the striker ISN'T restricted in its rearwards movement and see what happens........... It's all really beyond my level of technical and engineering expertise

[alterego]

Lol, ok. Setting the firing pin aside, what do you recommend? Headspace is good, just an ammo change? Anything look amiss in the FAL cases to you?

[Joe H]

Guys,

I have a Bren MK II semi I built. I've used a SA 7.62 barrel with a Bren bolt & L4 extractor. The mag well is modified slightly to accept the 8mm Bren mag which handles the 7.62 nato round well.

The firing pin is original Bren .303 with the extension hard silver soldered in a notch ground in the rear of the firing pin. Other than that I lightened up the firing pin spring to work with the semi striker.

As I remember, the fired cases show the rectangular pin indentation and some "primer flow" ridges but nothing like those originally posted by alterego. I'll scrounge around and see if I can find any. If I can I'll post pics. The significance being that my set up is an original FP - cross pin setup with a lighter spring.

I might expect to see some "primer flow", as even if the tip of the pin stays flush with the bolt face the tip of the rectangular pin is rounded and there is space for the primer to flow but nothing like alterego's original pics! I'm sure Mr E's analysis of aterego's problem is accurate from my experience. I've seen some .303 cases from FA that showed a slight amount of primer flow and some that showed none at all ???
Joe