OOB premature discharge question

[Peter Laidler]

Someones pointed out to me that if the gun had the ORIGINAL Mk1 type barrel nut, then it wouldn't have happened because that nut had a block to prevent for and aft movement of the piston extension if the nut were raised. But this block was removed during the first simplifications to the gun in 1939/40. On the basis that '.....with the proper and correct training, such an event (leaving the barrel nut undone) or using the gun without the barrel nut fitted never likely to occur'. The wise old trials engineers of 1940 didn't cater for enthusiastic amateurs though.....................

[mrspeel]

Back in the day, when it was legal to shoot the Bren in Canada, as a converted auto (ie semi only), I experienced a couple of issues with surplus ammo in an Inglis Mk1m. It was '5o's era 'GB' mk 7 ball, on stripper clips, in slings, pulled straight out of the case. While the ammo appeared bright and clean, it produced hang fires and short recoil/feeding failures. After I retrieved my cases, I noticed that the bren firing pin "indent"was now an "outdent". The metal was "standing proud" on the firing pin, like a square triangle.

What could be the cause? Primer, powder?

To quote Major LM Yearsley, from a UK May 1990 Guns Review article" if the firing pin can bounce, so can other moving parts of a weapon mechanism. This applies to the piston of the gas operated machine gun. This, in combination with hang fires, can produce a pattern of behavior which can lead to the user drawing quite misleading conclusions as to the cause."

The Major goes on to write that "In the early sixties, a complaint of weak ammunition was received from the Middle East, in connection with the .303 bren. Short recoil, due to insufficient gas, was suggested by both the user and the local investigator, as correct functioning proved to be impossible, even with the gas regulator set to its larger hole.The gun and a sufficient sample of the ammunition date of work in use, were sent home for investigation."

The Major reports that the reproof confirmed the ammunitions working pressure was within specification. The fired cases, both those returned, and those collected, showed "very severe piled metal, the deep impression of the bren firing pin standing, in many cases, almost as proud of the cap surface as it had originally indented".

The hang fires, piled metal, and short recoil, were explained by the following theory: "in effect, each cap had fired at the protrusion of the firing pin, but the duration of the hang fire was such that the bounce of the piston had permitted almost full withdrawal of the firing pin before the rise of internal pressure, which was normal, had forced the indentation in the cap back into contact with it. by the time the gas, tapped off further up the barrel, could reach the head of the piston, the latter had finished its bounce, and was traveling forward again, so its direction of travel had to be reversed by the expanding gas, instead of the gas accelerating a bouncing piston to the rear. This required a much larger volume of gas than could be provided by an port in the gas regulator."

It could be that the movie prop bren experienced a hangfire and a firing pin/piston bounce. The increasing pressure caused the unsupported primer to flow back/tear itself loose, to then release gas back into the breech. Another possiblity as you noted, is that a piece of the primer tore loose after round one, and then jammed itself into the firing pin slot, to then be rammed into the primer of round number two upon action closing.

It does appear that your issue is with the aged ammunition, not stored under ideal conditions. The use of recently manufactured ammo is probably your safest bet, as a person cannot judge primer quality on the basis of appearance alone.

[Brit plumber]

The hang fires, piled metal, and short recoil, were explained by the following theory: "in effect, each cap had fired at the protrusion of the firing pin, but the duration of the hang fire was such that the bounce of the piston had permitted almost full withdrawal of the firing pin before the rise of internal pressure, which was normal, had forced the indentation in the cap back into contact with it. by the time the gas, tapped off further up the barrel, could reach the head of the piston, the latter had finished its bounce, and was traveling forward again, so its direction of travel had to be reversed by the expanding gas, instead of the gas accelerating a bouncing piston to the rear. This required a much larger volume of gas than could be provided by an port in the gas regulator."

Are they saying that the piston/bolt carrier has bounced rearward the few thou required to let the piston post release the firing pin to be withdrawn due to the firing pin spring? And then before the return spring has had chance to close the piston/bolt carrier again the gas flow has cought up and forced the working parts to the rear? That was a real clever armourer who worked that one out.

[csmarcher]

Back in the day, when it was legal to shoot the Bren in Canada, as a converted auto (ie semi only), I experienced a couple of issues with surplus ammo in an Inglis Mk1m. It was '5o's era 'GB' mk 7 ball, on stripper clips, in slings, pulled straight out of the case. While the ammo appeared bright and clean, it produced hang fires and short recoil/feeding failures. After I retrieved my cases, I noticed that the bren firing pin "indent"was now an "outdent". The metal was "standing proud" on the firing pin, like a square triangle.

What could be the cause? Primer, powder?

To quote Major LM Yearsley, from a UK May 1990 Guns Review article" if the firing pin can bounce, so can other moving parts of a weapon mechanism. This applies to the piston of the gas operated machine gun. This, in combination with hang fires, can produce a pattern of behavior which can lead to the user drawing quite misleading conclusions as to the cause."

The Major goes on to write that "In the early sixties, a complaint of weak ammunition was received from the Middle East, in connection with the .303 bren. Short recoil, due to insufficient gas, was suggested by both the user and the local investigator, as correct functioning proved to be impossible, even with the gas regulator set to its larger hole.The gun and a sufficient sample of the ammunition date of work in use, were sent home for investigation."

The Major reports that the reproof confirmed the ammunitions working pressure was within specification. The fired cases, both those returned, and those collected, showed "very severe piled metal, the deep impression of the bren firing pin standing, in many cases, almost as proud of the cap surface as it had originally indented".

The hang fires, piled metal, and short recoil, were explained by the following theory: "in effect, each cap had fired at the protrusion of the firing pin, but the duration of the hang fire was such that the bounce of the piston had permitted almost full withdrawal of the firing pin before the rise of internal pressure, which was normal, had forced the indentation in the cap back into contact with it. by the time the gas, tapped off further up the barrel, could reach the head of the piston, the latter had finished its bounce, and was traveling forward again, so its direction of travel had to be reversed by the expanding gas, instead of the gas accelerating a bouncing piston to the rear. This required a much larger volume of gas than could be provided by an port in the gas regulator."

It could be that the movie prop bren experienced a hangfire and a firing pin/piston bounce. The increasing pressure caused the unsupported primer to flow back/tear itself loose, to then release gas back into the breech. Another possiblity as you noted, is that a piece of the primer tore loose after round one, and then jammed itself into the firing pin slot, to then be rammed into the primer of round number two upon action closing.

It does appear that your issue is with the aged ammunition, not stored under ideal conditions. The use of recently manufactured ammo is probably your safest bet, as a person cannot judge primer quality on the basis of appearance alone.

My first question when I arrived, was "What ammo were you shooting with ?" When I found out it was old 1950's stuff, I suspected something of that ilk had happened.

I supplied them with some modern privi-partizan ammo which cured the hangfire problem.

Thanks to all the Bren experts for their diagnosis of the problem
It is very much appreciated.

Gary

[Peter Laidler]

The breech block, piston and piston post in a Bren cannot bounce in the mechanical sense of the word.......... Shall I elaborate? It's going to get pretty heavy but....... For a start, it's nothing to do with the gas port or gas regulator setting OR HANGFIRES unless there is a hangfire and someone is quick enough to re-cock the gun.

At the moment of firing, the breech block is positively locked, up in front of the locking shoulder in the body. The firing pin is driven forwards by the piston post which is carried forwards under the weight of the stored energy in the return spring and the inertia of the mass of the piston post, piston and the piston extension (the PP/P/PE). At the instant of firing, the firing pin is still travelling forwards until it protrudes between .040" and .050". At which point the forward moment of the PP/P/PE comes to a DEAD stop. This sudden stop is caused by the front rounded face of the PP striking the internal front rounded face of the breech block.

This is the only thing that causes the PP/P/PE to stop. There is NO other feature that will cause the PP/P/PE to stop. Firing pin bounce or any other bounce is a red herring because the whole mass of the PP/P/PE is effectively cushioned by the PP/P/PE hitting the the striker which strikes the soft cap which coincidentally at the same time fires the round.

The Bren had a couple of failings but 'firing pin bounce' was not one of them or believe me, we'd have heard about it PDQ! Can anyone explain a Bren gun that operates on a short recoil system....... It's long stroke gas. Am I missing something somewhere or can someone point me in a safe direction?

What say you Tankie, Son and Skippy. Get your thinking caps on and EMER's out! Short recoil................................... I must be barking!

[MGMike]

From experience I venture to say that most alleged "out-of-battery" incidents , when fully investigated by competent examiners, turn out not to be "premature" at all. Rather, they relate to some anomaly that occurred AFTER an otherwise normal firing pin indent.

M

[tankhunter]

The breech block, piston and piston post in a Bren cannot bounce in the mechanical sense of the word.......... Shall I elaborate? It's going to get pretty heavy but....... For a start, it's nothing to do with the gas port or gas regulator setting OR HANGFIRES unless there is a hangfire and someone is quick enough to re-cock the gun.

At the moment of firing, the breech block is positively locked, up in front of the locking shoulder in the body. The firing pin is driven forwards by the piston post which is carried forwards under the weight of the stored energy in the return spring and the inertia of the mass of the piston post, piston and the piston extension (the PP/P/PE). At the instant of firing, the firing pin is still travelling forwards until it protrudes between .040" and .050". At which point the forward moment of the PP/P/PE comes to a DEAD stop. This sudden stop is caused by the front rounded face of the PP striking the internal front rounded face of the breech block.

This is the only thing that causes the PP/P/PE to stop. There is NO other feature that will cause the PP/P/PE to stop. Firing pin bounce or any other bounce is a red herring because the whole mass of the PP/P/PE is effectively cushioned by the PP/P/PE hitting the the striker which strikes the soft cap which coincidentally at the same time fires the round.

The Bren had a couple of failings but 'firing pin bounce' was not one of them or believe me, we'd have heard about it PDQ! Can anyone explain a Bren gun that operates on a short recoil system....... It's long stroke gas. Am I missing something somewhere or can someone point me in a safe direction?

What say you Tankie, Son and Skippy. Get your thinking caps on and EMER's out! Short recoil................................... I must be barking!

Peter,You put it quite clearly & Sucinctly. I can add nothing from my Armourers expierience & knowlege, that you havent already covered. As you state, 'We WOULD have heard about it'! Others here on the forum may possibley be unaware of how the system works in Service with our Tech publications available to the Armourers & all R.E.M.E Tech Personell. IE> E.M.E.R's, A.E.S.p.s, I.S.P.L,s Etc.
For the uninitiated: One of the things we expirienced as Service Armoures was Imeaidate instructions & also Miscellanous Instructions. These were sheets forwarded to units to insert into our Engineering Instructions Manuals (E.M.E.R.s) for us to execute any modifications or effect a different style of repair. This alleged Firing Pin probelm would have been an 'Imediate Mod' & If of such a dangerous situation. The Mod Inst Sheets would have been proceeded by a Signal sent out to ALL units in the Armed Forces. Even overnight if deemed nessacary, so that all equipment could be quarantined for safety & not used until the mod had been executed. Or the equipments inspected for faults/ defects or indeed the mod kits fitted.
All the 'Blurb' from that 'Officer' in the posted report, was interesting to see. BUT, as you have clearly pointed out a load of old B****ks! MOST Officers in almost ALL Senior Armourers Expierience, are desk wallahs & have virtually no indepth Technical 'Hands on' exposure to working on equipments (Peter is ONE of a few exceptions here, as he has climbed up through the ranks from a Humble Craftsman (Private Soldier) to Warrant Officer level. And then through his Tech abbilities & immense skills, been Commendably selected for promotion through Commisioned Officer Level) a LOT of Officers seek to gain attention to thier careers by Writing reports to get themselves a name & seek to become 'Acknowleged Experts' for this path of chosen action. In a LOT of cases, they certainly DO get themselves a name, but unfortunately, it's not really one they were originally seeking!.........LOL! The Bren/ LMG is one of THE most relaible & Battle proven weapons of our time. A LOT of time has passed since it's original introduction, & also the bugs ironed out until it has perfomed with legendary reliability & affection. Anything connected with Breech expolsions or other failures after all this elapsed time is USUALLY associated with Ammounition. You cannot get better praise then from Armourers, or indeed the intended End User. I have heard nothing detrimental spoken of a derogatory nature from the Good Old Infanteer. However, I have heard a LOT of derogatory things about SOME Officers!............LOL! Peter, I KNOW you will take this in the spirit it is writtern, & also be aware that this 'Comment' is certainly NOT directed at YOU! LOL! Cheers.

[MGMike]

I have some Pakistani .303 belted for Vickers that routinely produces horribly extruded primers when debelted and fired in a Bren. The problem disappears completely when decent ammo is substituted. Greek HXP and Norma military come out looking perfectly normal.

So much for bolt bounce.

M

[tankhunter]

I have some Pakistani .303 belted for Vickers that routinely produces horribly extruded primers when debelted and fired in a Bren. The problem disappears completely when decent ammo is substituted. Greek HXP and Norma military come out looking perfectly normal.

So much for bolt bounce.

M

Mike, It is a known fact that OLD/Stale Ammo produces high & unstable pressures at differing tempretures. Resulting upon occasison in much higher breech pressures. This can possibly also lead to primer caps blowing out of the base of the round. You may possibly be aware that bren Breech Block faces were later modified with TWO holes in the front cartridge recess. It was for this very problem, to direct the resultant gases from the round away & sideways from the users face & the potential for excessive force. To drive the working parts rearwards & consequent damage or injury! Some cases have also ruptured due to this ageing problem with old Ammo as well. The metal structure of the brass changes with age which does not help with higher pressure involved! As you later discovered, fresh/newer production Ammo cured the problem! In general terms, there is enough 'Meat' around the sides & breech area in the magazine vercinity to enable the gun to withstand a breech expolsion. or ruptured case & resultant 'Blowing out' of gas/Metal downwards through the ejection opening & upwards out of the mag opening to direct this unexpected force away from the user. But, If you have ever expirienced actual breech explosions, they MOSTLY all differ in small ways due to differing defects occuring at a particular time. BUT, this does NOT detract from the fact that you REALLY dont want to have one if it can be avoided! You only have one set of hands, eye's, Etc, Etc.........

[Peter Laidler]

A few years ago I saw that we were using some old RG-69 7.62mm belted ball in our GPMG's. I spoke to the ammo techs, in passing, who said that it was old war stocks being rotated and used for training. Waste not, want not. But it goes to show that it's not the AGE that matters too much but how it's been stored