54R Bren Issues

[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

[djandj]

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

Many thanks Mr. E. You are quite correct. It appears that my rifle is not headspacing with the cartridge at all (see pics) The bolt stops with the bolt stops which are also aligned with the end of the barrel. The back end is held in by the shoulder stops. So shimming the barrel will not change anything for me (right?) It would appear that if we want to have a bit a play in the bolt back and forth between the stops (and barrel) and the Locking Shoulders we need to get a shorter licking shoulder (right?)

Here we see the bolt resting directly against the barrel an the stops


Here we see that even with the cartridge, the spaces between the barrel and the bolt face etc. don't change. The casing fits in the pocket created by the stops and bolt face.

[ActionYobbo]

in both pictures it looks like the bolt is stopping on the barrel. can you fit a slice of business card between the case and the bolt face?
to me the issue looks to be zero head space...
back to Joe H

PS Joe is a guru when it comes to semiauto brens. he has helped me a bunch over the years

[djandj]

in both pictures it looks like the bolt is stopping on the barrel. can you fit a slice of business card between the case and the bolt face?
to me the issue looks to be zero head space...
back to Joe H

PS Joe is a guru when it comes to semiauto brens. he has helped me a bunch over the years

Quite right Mr. E. The bolt contacts directly on the bolt stops and barrel = 0 headspace since the locking shoulders allow for NO rearward movement of the bolt when locked in the shoulders.

[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.

[Joe H]

Djandj,

Here is a simple way to check headspace on a Bren. The rim thickness on the 7.62 is .063" so you can use the same headspace dimensions as the .303. The Bren was never designed for the 7.62x54r so I'd keep the headspace as low as possible.

Strip the frame, carrier and bolt. Remove the firing pin and extractor from the bolt. Take a properly resized MT case or unfired 7.62x54r case with the powder and bullet removed. The 7.62 x 54r is within .001" of the .303 british rim so you can check using the .303 headspace dimensions. Check the rim thickness. It should be about .063". It is a little difficult to check since the head is not flat. Glue on a .003- .004" piece of shim stock on the head of the case. Slide the case in the barrel. Now only install the bolt without extractor, no carrier spings etc, only the Bolt. Push it into the locked position. Push the bolt forward with your finger. The case should be snug under the bolt. If it isn't check Item 3 above. With your finger still pushing on the bolt and using a feeler gage you should have between .002" and .005" between the bolt and locking shoulder. If its .010" or better you may have problems. If its .015" plus IMO you definitely will have a problem. You probably will have to replace the locking shoulder with a longer one.

Every time the bolt closes the relationship of the carrier and bolt should be as in the attached pic. The carrier should have to travel 3/8" to 1/2" before the bolt unlocks. If you don't you may have problem 2 above.

The second pic shows the gap between the bolt and locking shoulder on a Bren that was having the same problems as you.

Joe

3-The bolt is hitting the two lugs on the receiver before it closes on the cartridge. Could happen but not as likely as the two above.

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

[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.........