Semi Bren Primer Flow

Guys,

Was shooting my L4 today and have some primers I am concerned about. Ammo used was factory Lake City 7.62x51 nato stuff. I am not getting flattened primers, but it does look like I am on the very verge of piercing the primers. Looks like there is a sheared bit of metal corresponding to the firing pin geometry that just hasn't liberated itself yet. Firing pin protrusion is 0.050" (1.27mm). It would seem to be a bit great in light of a rule of thumb I have read that one doesn't want to have firing pin protrusion much in excess of 1/2 the pin diameter which I'm using the narrow section of 0.078"..so it's ~0.015" greater than that.

Narrow section of pin to bolt channel clearance is ~0.003-0.004".

Something to bear in mind is that the semi firing pins are heavier on account of the length to get to the striker. Not sure how hard the striker is hitting it in comparison to the carriage post thingy in a full auto gun.

Anyhow, was going to start working the pin protrusion down until more normal primer conditions prevail.

Please let me know what you guys think, thank you in advance for any help...

I think you're worrying about nothing. If you aren't getting pierced primers then don't worry. The hole should appear in the dent of the primer anyway. The Bren FP has always nearly come out the mouth of the case. If you were shooting reloaded ammo then there might be an issue but this is factory. Don't worry about what hasn't happened...

I'll try to put some pics up. Something looks clearly amiss to me...

Pics attached

Possibly enlarged firing pin hole...so it appears.

Holy shxx! That (photo 1) looks to me like........... Are you putting TNT in your reloads? The pressure in the case is forcing the primers back into the striker hole. Is the gun a standard UK made L4 or a US re-weld/semi auto? While I don't know much about the US rewelds it LOOKS like the primer has blown right into the striker hole, driven the striker right back against its stop, THEN the primer has tried to extend further into the hole, wrapping itself around the striker to the point of almost exceeding the extent of its elasticity ALMOST ready to pop.

Never seen anything like it in my life and I've seen one or two Brens and L4's. You MUST compare and measure the striker hole in the breech block with a know good one. Is yours enlarged? It will be before long!

Just my view as a bit part onlooker.......

Pin to hole fit looks good. I do not think its over pressure. Note primer isn't flat. Will double check headspace but don't anticipate problem there

Quote:

---

Originally Posted by alterego

Pin to hole fit looks good. I do not think its over pressure. Note primer isn't flat. Will double check headspace but don't anticipate problem there

---

Can you post a photo of your semi auto firing pin and your breech block? Are you using an L4 BB and a South African barrel or a a South African BB with the South African Barel?

303 bb, SA extractor and barrel. I've got a test I'll run tonight. I think I know what might be happening

Cutting to the quick, if I might be so bold and with the utmost respect, but as an Armourer of one or two years standing and a graduate mechanical engineer, if that ain't over pressure what's caused that, for whatever reason, pray tell me what it is. I could be wrong of course as my wifey tells me most days. But whatever it is, can I please have one of those fired cases?

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

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.

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.

https://www.milsurps.com/images/impo...DSC02623-1.jpg

Quote:

---

Originally Posted by Peter Laidler

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.

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).

Quote:

---

Originally Posted by alterego

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...

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

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.

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.

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

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.

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

Quote:

---

Originally Posted by Mr E

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.

https://www.milsurps.com/images/impo...ing20pin-1.jpg

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.

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.

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

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.

Quote:

---

Originally Posted by alterego

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

---

This ought to be good...

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

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?

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

The end of that firing pin looks odd, is it completely squared off?

Quote:

---

Originally Posted by Brit plumber

is it completely squared off

---

It appears to be new and I should think subject to fitment and adjusting.

Guys,

I found some 7.62 Nato cases I fired in my Bren. Ridges do not extend past the base of the case.

Joe


Attachment 85515

Those look absolutely correct...

Did u "tune" your ejector?

Quote:

---

Originally Posted by alterego

Did u "tune" your ejector?

---

Alterego,

Did nothing to the ejector. Why would that have anything to do with the "primer flow"?

Firing pin is the original FA pin with an extension added on. No change to the cross pin. FP spring lightened up for semi.

Joe

https://www.milsurps.com/images/impo...3_500gif-1.jpg

this is us waiting for an update

Quote:

---

Originally Posted by Mr E

us waiting

---

Sort of typical though...

That looks like a group of Armourers on a range day with GPMG's. Waiting for something to happen or go wrong. It never does!

Quote:

---

Originally Posted by Peter Laidler

GPMG's

---

So, back up to the 1919A4s then, lots of work then. At least when we had ours out there was. Guns would be running back and forth from line to truck and the armorers would be working non stop...not the C6s though.

Quote:

---

Originally Posted by alterego

Yes, it's Mighty Lime's (an individual who relishes using damned Bren guns to hurt my feelings) most excellent setup.

---

Hmmm, this is the look on Mighty-Lime's face (left) when Alterego (right) is toying with his Bren...

https://www.milsurps.com/images/impo...traitpng-1.jpg

...and here we have the look on alterego's face (right) whilst Mighty Linus (left) is toying with his Bren gun without his security blanket...

Yes, I had that happen with a Canadian 8mm Bren and a handload of questionable consistency...shocking. Best part is, the gun isn't hurt at all I bet.