54R Bren Issues

[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

[Vincent]

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?

Not quite. A .303 bolt would have to be modified to fit.

[Peter Laidler]

Nope...... the massive build-up of gas is instantaneous and it might have blown downwards UNTIL it will have met the top of the piston extension where the extractor cam is situated. From there, there'd be nowhere else to go and as a gas always takes the line of lease resistance it was deflected elsewhere........... straight into the body and magazine

[Joe H]

Peter,

Every head spacing number I have given DJ is consistent with the Bren Gun Manual which is for the .303 British. Exactly what I have used for my Mk I & MK II semi builds. We are all saying the same thing about the receiver stops. The bolt should bear on the cartridge head not the stops. If, as we suspect, the bolt stops on the bolts stops and has no clearance between the bolt and LS it is clearly not set up correctly. Both cartridges are rimmed and are within .001" of the same rim thickness.

The bolt for the .303 if the rifle is head-spaced properly should work for the the 7.62. The only modification made to the bolt is machining a larger dia for the 7.62 rim. The bolt face should not be altered as Wally G noted. IMO the 7.62 should be set up with higher headspace standards than the .303 as the .303 case head makes it far more tolerant of loose headspace than the 7.62.

Until DJ tells us how much space is between the case head and the bolt face we really don't know if it is an issue. If its .005" the gun is not set up properly but headspace is not causing his problem. If its .015" then it is probably causing his problem.

I guess I still haven't made it clear regarding what my "out of battery" issues are. In the original FA Bren after the instant of firing the carrier has to travel about 3/8" -1/2" before the carrier unlocks the bolt. The instant the bullet passes the gas hole in the barrel the impulse from full gas pressure drives the carrier rearward. IMO the 1/2" -3/8" of travel is to allow the bullet to leave the barrel and barrel pressure to dissipate before the bolt unlocks. If the carrier is not fully forward there may still be gas pressure when the bolt unlocks. I have seen semi striker designs that could allow this to happen. The combination of drag on the return so that the carrier is not fully forward and a striker design that allows the rifle to be fired without adequate carrier travel could cause case blow outs IMO. This could never happen in the FA Bren

Joe

[djandj]

Djandj,

Is this what is happening???? Use the feeler gages and determine what the amount of space is between the rear of the case and the face of the bolt which is stopped on the receiver lugs? Quote from above " If its .010" or better you may have problems. If its .015" plus IMO you definitely will have a problem." The bolt should bear on the cartridge case. The receiver lugs (stops ) are there to prevent the bolt from battering the barrel.

If your assessment is correct and the bolt is stopping on the receiver lugs before contacting the case just as Mr. E posted the gun cannot be properly head-spaced. In the current configuration the operational clearance which should be .002" + is zero.

Joe

Thanks Joe. As you can hopefully see in the pics, everything is lined up for an exact fit. There is NO space between the bolt and the stops (except along the upper part looks like I can get the .002 gauge in a little way - but then it stops further in) and No space between the cart. and the bolt. As near as I can figure it, the pocket formed is exactly the size of the round. (unless of course the round is out of spec...) Once locked in the shoulders, there is NO movement possible forward or back by the bolt.

[djandj]

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.........

Thanks Peter. I checked again and as near as I can tell there could be possibly be .001 space between the end of the cart. and the the face of the bolt. (can't get the gauge in that small an area).

As for steel failure... well I suppose it's possible, but the steel of the casings which fails seems rather thick. Much thicker and tougher than a similar brass cartridge. If headspacing isn't the issue, and the MFG was right and it was the steel then why did the problem change when I sent it back to the MFG the first time. I sent it back b/c it was crushing the recoil springs as well as blowing out the shells. He replace the barrel and perhaps some of the internal parts. However with the old barrel and parts when they blew out, it did not affect the operation of the rifle and did not blow out the mag etc. Now when this happens (every 60 rounds or so), we get a massive amount of gas shot back through the receiver enough to blow out the steel mag and make it look like a piece of pop-corn. Ammo stayed the same.... changed parts in the gun and the problem changed. Logic would dictate that this is at least partly influenced by the gun. (no?)
Again, should there be a bit of movement forward and back by the bolt when locked in the shoulders? My bolt is absolutely locked against the bolt stops to the front and the shoulders in the back.

Thanks,

[Vincent]

What's the next step(s)? A shorter locking shoulder and one of Joe H's strikers?

[Joe H]

Djandj,

As I see it, assuming all your info. is correct, your Bren is far too tight. You should have .002" or .003" Above the spec rim thickness for headspace and .002" min for operating clearance. I'll throw this up for other comments. I would take a few thous off the bolt stops and .004" to .005" off the locking shoulder.

Think about this: With no operating clearance between the bolt and LS as you fire 60 rds the bolt becomes hotter and harder to close causing the carrier problems moving forward. The sides of the receiver also get hotter and hotter but they can dissipate the heat faster. Just a thought

Joe

[djandj]

Djandj,

As I see it, assuming all your info. is correct, your Bren is far too tight. You should have .002" or .003" Above the spec rim thickness for headspace and .002" min for operating clearance. I'll throw this up for other comments. I would take a few thous off the bolt stops and .004" to .005" off the locking shoulder.

Think about this: With no operating clearance between the bolt and LS as you fire 60 rds the bolt becomes hotter and harder to close causing the carrier problems moving forward. The sides of the receiver also get hotter and hotter but they can dissipate the heat faster. Just a thought

Joe

Thanks Joe - definitely along the same thinking that I had. It's just too tight in there. The bolt has NO movement in it and is resting directly upon the bolt stops and the shoulders. There is really no space to mention between the back of the cart. and bolt face. It APPEARS to me that there is essentially a zero headspace.

So.... who wants this monster.... Whom can I send it to for the proper headspacing and detail work?