54R Bren Issues

[Joe H]

djandj,

From your description your headspace is far too tight (nonexistent) just as Tankhunter posted. I didn't see any shims on the case head so if you add the .003" shim I suggested plus the minimum .002" between the bolt and LS without the shim you should be reading .005" between the bolt and LS. To explain the numbers Tankhunter mentioned and I think he means .064" to .074". The case head is theoretically .063". So if the bolt could only move .001" then the headspace would be .064" far too tight for operation of military firearm. I suggested .063" +.003 +.002= .068" Bolt can move .005" (.068"-.063"). The upper level is .074 - .063=.011 that the bolt can move. The .074 is commonly called a Field gage. If the firearm exceeds this the rifle is removed from service and repaired. You do not use the Field gage to build a new rifle.

Peter's .6" range numbers are referring to the amount of carrier travel after lockup and before the FA Bren fires, I think.

Peter,

Depending on how the striker is set up it is possible for the semi Bren to be fired from I what I'm referring to as an "out battery condition", that is the carrier is not fully forward (but the bolt is locked) and short of the same position it would have to be in for the FA Bren to fire. The firing hammer on FA Bren is the piston post, which is part of the carrier. It physically cannot hit the firing pin unless the carrier is in the correct fully forward position which allows about 1/2' of travel before the bolt unlocks. In the semi the hammer (striker) is totally separate from the carrier and depending on it's configuration it can hit the FP and fire the gun without the carrier being in the same position as it must be in to fire in FA.

Dj,

Wally G is correct about messing with a locking shoulder. BRP is a great outfit to deal with and is the only source of various sizes of locking shoulders. I screwed one up and had to make new one out of steel that could be hardened. I have ground them back very carefully using a disc grinder and a homemade jig to assure the correct angle. A file probably won't work. You need emery paper and a flat surface like a piece of plate glass. .005" is a lot to remove by hand. You must maintain the correct angles. A sketch I made of one is attached. I've used a micro air pencil grinder to remove the peening on the LS. A dremel should work.

Be sure you understand what is being suggested and what you have to do before you do anything to your locking shoulder. Best for you would be as Wally G suggested and get a new one .005" smaller and try it out. You need a dial caliper to measure what you have (overall length) to tell Brian what you need. I don't think the numbers on the LS mean anything. The armorers would just sort through the pile until they found one that worked. I'm sure Peter can comment on that.

I've never used the barrel shims Wally suggested. If you try this make sure the bolt does not end up bearing on the receiver stops before touching the cartridge.

Joe

Information

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[djandj]

Thank you Joe and Peter. I absolutely will listen to the experts. So it would appear that I need about .005 worth of HS added. So, as I understand it, it's either get a shorter LS or barrel shims. The ones I see listed at BRP are .002 (Bren Barrel Shim) so a bit tough to make .005 out of those. Do they come in different sizes? I couldn't find them on the schematic at BRP. where do they go? Looks like they are easily reversible and perhaps even easy enough for me to install.

If I understand, it looks like the shim would keep the barrel from going as far back into the rifle as it does now thus created HS. Are there other issues to address if the barrel is shimmed and thus not going in as far as it does now?

Thanks again guys.

[ActionYobbo]

before you go grinding and cutting...
do you use the gun in 303?
if so is the HS correct for the 303 barrel?
if you answer yes to these questions then modify the 7.62 barrel by facing off the breach and running the chamber deeper. there is no sense in modifying the existiing HS if its correct for one of your barrels

also from your pictures I cant tell if you are headspacing off the case or the bolt stops.

[djandj]

before you go grinding and cutting...
do you use the gun in 303?
if so is the HS correct for the 303 barrel?
if you answer yes to these questions then modify the 7.62 barrel by facing off the breach and running the chamber deeper. there is no sense in modifying the existiing HS if its correct for one of your barrels

also from your pictures I cant tell if you are headspacing off the case or the bolt stops.

As for HS - I did it with and without the cart. in the barrel also with and without the extractor (and shell) and the bolt locks in with NO movement either way. (what does that mean?)

As for grinding and cutting, I'm NOT going to do that. I'll either replace the locking shoulders or use a barrel shim as those steps are reversible. I'm not qualified to be cutting or grinding anything on this rifle (other than the wood perhaps)

[ActionYobbo]

As for HS - I did it with and without the cart. in the barrel also with and without the extractor (and shell) and the bolt locks in with NO movement either way. (what does that mean?)

As for grinding and cutting, I'm NOT going to do that. I'll either replace the locking shoulders or use a barrel shim as those steps are reversible. I'm not qualified to be cutting or grinding anything on this rifle (other than the wood perhaps)

you need to determine what is actually stopping the bolt it should be the case head but if its the bolt stops then you cant set head space
headspace is measured between the bolt face and the cartridge so stick in a dummy case and shut the bolt with out the extractor and see what gap you have between 1) bolt face and cartridge case 2) bolt and barrel 3) bolt and bolt stops and report back
with a case in and the bolt shut the bolt should not be touching 2 or 3
the bolt stops are only there to stop the bolt from flying out when the barrel is out and to stop the bolt from peening the barrel when the bolt is dropped on an empty chamber. they are not involved in any part of live firing

[MGMike]

What leaps out at me is the long casewall rupture at a point where it is --or should be-- supported. I suspect faulty brass.

M

[djandj]

Mike, the casing are steel (not brass). And they don't all fail. Just one in 60 or so. It appears to be some variance in the operation of the rifle. With essentially a zero headspacing, the current thought is that if there is ANY discrepancy in the cartridge or any unburnt powder or other fowling, there is no room for error possibly resulting in the rifle firing out of battery.

[Peter Laidler]

Whoooooooa lads....... Let me stop you all there! The depth of the cart seating in the breech face of the '303" Bren gun is .055" to .059" and the MINIMUM acceptable CHS is .064". The breech block must NOT touch the breech block stops (the BBS's) when a round OR CHS gauge is in the chamber. This difference (between .059 and .064 is the reason why, if you use barrel washers to bring the barrel forwards a tad, you can only use two, up to a max of .006"..... get it?)

The purpose of the bbs's on the .303 gun are to STOP THE BREECH BLOCK from slamming into the face of the barrel when it's dry fired. This is WHY, when the breech block is closed on an empty breech, there is a running clearance between the rear of the breech block and the locking shoulder of (forget it now but it'll be something like .005" or so.....) But you get my drift!

What I am minded to suggest DJ does is set his gun up to the old Bren CHS of .064" GO and .074" NO GO by changing locking shoulders to suit. I am sure that the 762x54R CHS won't be far removed from this! Rimmed cartridges are more tolerant to CHS variables for many reasons*.

But while CHS is under discussion in this thread, I'm not convinced that it is the cause of the blow-jobs - or blow-outs! I'm convinced that they are caused by steel case failure every so often.

Nor am I convinced that the gun is firing with the breech partially closed. Guns that have had genuine breech explosions, such as SA80's and the odd GPMG, caused by user error, have catastrophic failures due to the explosive effects of the full load. As opposed to DJ's where the magazine blew off due to what is best described as a severe leakage of gas. In fact I mention a classic case regarding a Bren in 'the book' which I am now actively pursuing as we speak.....

Even for a US spec s/a Bren to fire on an open or unlocked breech would be a s/a Bren manufacturers legal nightmare - or a lawyers lucky day - depending on where you stand of course. You know what they say........... If you don't make it foolproof, a fool will prove you right. It's called Murphys Law

Nope, CHS needs to be put right but with regards to the failure it's just a mildly contributory factor and we're getting well into mission creep in the meantime.........

[WallyG.]

If I'm understanding this statement properly... shall we consider one other possible variable in our equation?

"The depth of the cart seating in the breech face of the '303" Bren gun is .055" to .059" and the MINIMUM acceptable CHS is .064"."

As I understand it... The Breech block has also been machined to accept the 7.62 case head in this conversion... if it was inadvertently bored deeper and wider (wider being the only required modification for the conversion scenario - for the cartridge rim) would not replacing the bolt improve our dimensional conundrum? I'm wondering if the cartridges are seating some extra depth in the breech block face and allowing the breech block to contact the BBS.

A .303 barrel and .303 bolt and some locking shoulders is all that Djandj needs to set up the CHS per the last post's instruction - right?

[ActionYobbo]

the described blow out was at the extractor but it blew the mag apart. extractor is on the bottom and mag is on top. the only way it could blow the mag off is if the breach was open.

this could also be caused by the floating firing pin being too heavy and firing the primer while the bolt is still moving forward