54R Bren Issues

[Vincent]

Peter,

I think you nailed it with your “blow-back mode” comment.

Back in post 93 djandj said there was almost zero CHS and the gun would fire with the breach block NOT completely locked.

If the breach block is not locked, it’s logical that the gun would be operating blow-back mode and that is the cause of the “premature extraction.”

The mechanical safety doesn’t prevent this from happening with this semi-auto striker system design.

[TactAdv]

Well, you could conceivably get any number of problems that would cause the gun to fire OOB, in a form of API. Chamber obstructions, stuck or jammed FP, etc., all could cause enough primer contact to set off the cartridge before full dwell is reached. All manner of other semi-auto gun types exhibit this all the time.

One of the things I never liked in all these "semiauto machine gun" design conversions like the BREn, RPD, DP, etc is the necessarily very long FP's that are employed......lots of room to get stuff in there and cause friction or outright jams and these all have very long pin protrusions to begin with. If the FP tip is not fully retracted as intended.....for any reason.....coming round again it can easily function as a fixed FP, especially if the primer cup thickness is not very....thick, or you are dealing with an old and over sensitized priming composition.

Might not be a "steel versus brass case", argument there at all, might just be a very differing primer sensitivity between the two batches of ammunition, coupled with either a sticking FP, or the reverse situation, a long, heavy FP that is allowed to slam forward under inertia??

-TomH

[Joe H]

Unless the gun is working in blow-back mode, the bullet has to pass the gas port in the barrel before ANY rearward movement of the BB&PAssy can take place. And even then at the 60:1 ratio, the Mech Safety feature ensures that the bullet has long left the barrel WELL before the BB has unlocked.

I realise that I don't truly understand the semi 'rifles' so please feel free to berate me in any way you like. I take it all in good heart..........

Peter,

Your statement above is absolutely correct for a Bren as they were originally built even in semi auto mode. The Bren is an open bolt MG and it is fired from an open bolt in either FA or semi. The piston post is the hammer that strikes the firing pin in either FA or semi mode. Since the piston post is part of the carrier it cannot touch the firing pin until the carrier is about 3/8" forward of where it unlocks the bolt. IMO the 3/8" is the distance the carrier should travel before the bolt unlocks allowing time for the bullet to leave the barrel and the pressure to drop before extraction. I have no technical proof of this.

In the US we cannot build an open bolt gun, it can be too easily converted to Full Auto (ATF Requirement). The striker fired semi auto (djand's Bren) separate the firing from the closing of the bolt so that the semi Bren becomes gun fired from a closed bolt.
After firing the gas piston operates the mechanism the same as the original Bren except that it has more recoiling parts to move (the separate striker) and it has to cock the striker spring. The return of the carrier & bolt may also have to deal with issues caused by misalignment as a result of the welded up receiver pieces. All our semi receivers are built from re-welded de milled Brens.
If on the return stroke the carrier stops short, for whatever reason, of the 3/8" forward of the bolt unlocking position and the rifle can be fired from this position IMO the bolt may unlock before the gas pressure has dropped to safe level. Or maybe the bolt never locked but in that case I think the results would be more catastrophic. The striker can be configured to eliminate this possibility by the lower part hitting the carrier if it is too far rearward and the upper tab not striking the firing pin. The takes both an offset striker tab and corresponding proper length firing pin to achieve this mechanical safety.

If Djandj's Bren can be fired without the 3/8" carrier travel it's my theory that he may be getting premature extraction. At least this is one thing to look at.



Joe

[Vincent]

Well, you could conceivably get any number of problems that would cause the gun to fire OOB, in a form of API. Chamber obstructions, stuck or jammed FP, etc., all could cause enough primer contact to set off the cartridge before full dwell is reached. All manner of other semi-auto gun types exhibit this all the time.

One of the things I never liked in all these "semiauto machine gun" design conversions like the BREn, RPD, DP, etc is the necessarily very long FP's that are employed......lots of room to get stuff in there and cause friction or outright jams and these all have very long pin protrusions to begin with. If the FP tip is not fully retracted as intended.....for any reason.....coming round again it can easily function as a fixed FP, especially if the primer cup thickness is not very....thick, or you are dealing with an old and over sensitized priming composition.

Might not be a "steel versus brass case", argument there at all, might just be a very differing primer sensitivity between the two batches of ammunition, coupled with either a sticking FP, or the reverse situation, a long, heavy FP that is allowed to slam forward under inertia??

-TomH

I understand those issues, but this makes me think it’s not one of them.

WOW Joe - you are way ahead of me on that. Yes, the gun does blow up usually on the first round of the mag (after manual racking).

It’s happening with the first round, when the cocking handle, slot, etc, limits how far the breach block and carrier can be pulled back. This in turn limits the amount of energy stored by the return spring.
But when a round is fired the breach block and carrier travel back a greater distance and more energy is stored in the return spring. So when the breach block and carrier move forward again they are pushed with more force than cocking can provide to lock the breach block.

[Joe H]

It’s happening with the first round, when the cocking handle, slot, etc, limits how far the breach block and carrier can be pulled back. This in turn limits the amount of energy stored by the return spring.
But when a round is fired the breach block and carrier travel back a greater distance and more energy is stored in the return spring. So when the breach block and carrier move forward again they are pushed with more force than cocking can provide to lock the breach block.

Vincent,

Additionally Djandj is using some sort of plug in his mag since he is in California and mag capacity is limited to 10 rds. Since it is a 25 rd zb39 mag he would have to have a plug equivalent to 15rds so that his 10 rds are rd 25 to 15. The mag may be causing enough drag so the the bolt doesn't lock properly. Easy enough to check. Don't use a mag & see what happens.

Joe

[Vincent]

Good point, Joe.

He could also flip the gun over after loading the first round from a magazine and see if the bolt is up and locked before pulling the trigger.

From what I have read about the Historic Arms semi-auto Bren guns, it seems to be a common problem for them to be very “tight.” My read on this (I could be wrong, so anyone who knows please chime in) is the receiver sections were not perfectly aligned when they were welded together. If that’s the case, there would be considerable drag on the carrier and that along with the zero CHS might be the problem.

--------

However........ In a Bren, as the breech block is drawn down by the piston post to unlock, the extractor cam tightens up on the extractor and effectively makes it a solid part of the breech block. (it's not of course but is rigid to the breech block. Try it out of the gun to see what I mean.....) Simultaneously, as the breech block is dropping down while unlocking, it is rotating about the lowest point of the front face of the breech block against the breech block stops in the gun body. This draws/rotates the cartridge seating face slightly rearwards. This action just tweaks the thick rim of the old just fired and red-hot 303" case and breaks the taper seal of the case in the chamber. This is the primary extraction in the Bren.

Very interesting post, Peter. The rotation explains why the rim looks like it has been twisted.

Attachment 62860

[WallyG.]

In what dimension are you reporting systematic misalignment of receiver sections during weld-up? Define misalignment please.

That is a rather strong defect to be called out on here say... I have a good many of the HA guns and all welded mating surfaces on mine are flush and level... I know first hand that a jig is used for positioning during welding... and that the finished joints (internal and external) are machined flush with a milling machine. But I suspect that is not what your saying...

Using a jig etc. would not necessarily prevent some minor receiver warping due to heat and variable contraction... but I know heat sinks are used and applied weld beads are alternating short runs to minimize over heating and receiver distortion. I would not define the effects of variable cooling and heating expansion/contraction as a "Misalignment" defect. This is just the harsh reality of re-welding torch cut steel receivers - to be successful one must control and compensate for the inevitable expansion and shrinkage inherent in the process. When warping does exceed tolerance... you have to cut the sections/segment of weld apart and start over... or scrap the entire receiver. Someone who knew what they were doing would have to evaluate this receiver to see if it was in fact square and concentric about its center line/axis. If warped... function problems like these could result.

Since you are suggesting that a warped receiver could bind upon the reciprocating internals... intuitively this would make perfect sense to me... but over time I would think that this binding would lessen as the moving parts wear in... and any real/material binding forces would routinely prevent the loading and extracting functions which appear to be taking place successfully in this gun for many rounds fired. If the warping was truly excessive... the internals would not function at all... they would simply bind up. Such a gun should not have left the shop in the first place... as it would not pass a final test function firing routine.

As for the charge of systematically tight headspace in HA Brens... I have no knowledge of this regarding HA production methods... seems plausible... and I have not had the need to go about measuring my guns... but it would be an interesting sample to test as they were produced over the last 10 years - some many years apart. This gun being discussed certainly needs more CHS to be in spec - no doubt.

Ponder this... if as you postulate, the front gas cylinder is not in alignment with the gun's axis due to receiver warping... the gas piston might be hanging up/binding on it's last few millimeters of forward travel in the gas cylinder... especially with carbon buildup after many rounds fired and thus potentially preventing bolt lock-up... but if this is the case... the occurrence should be more often and much more regular... and progressively worsen with increased fouling.

However, much of this is just observation and speculation... only the gunsmith that has the weapon in hand is in a position to efficiently evaluate its function or malfunction. Yet, I've thoroughly enjoyed the mental exercises that these discussion points and counter points have provided.

[Vincent]

In what dimension are you reporting systematic misalignment of receiver sections during weld-up? Define misalignment please.

I am not reporting it, other people are. Google and see for yourself. If I understand them correctly, it’s the x and y axis. X being the horizontal and y the vertical.

As for the charge of systematically tight headspace in HA Brens...

Read my post again. There’s no such charge. The owner of THIS gun says it has zero CHS.

Yet, I've thoroughly enjoyed the mental exercises that these discussion points and counter points have provided.

Let’s keep it enjoyable then. I understand you have several HA Bren’s and like them. But there’s no need for you to try and put words into my mouth that I haven’t said.

[djandj]

I thought the Yugo brass case was heavy ball and his steel case light ball?

Here’s the picture of the ammo can from post 48.

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


Am I reading the can wrong? 7.62, Light Ball, copper washed steel case, silver = steel core bullet, powder and ammunition produced in 1975, 440 rounds?

Just to add a little to the brass/steel debate. These steel cases have a copper coating, with maybe some zinc mixed in too. The coating acts as a kind of lubricant making the cases less likely to get stuck in the chamber.

Vince - that was the STEEL I was using. I later got the Brass cased Yugo and reported back with that.

[djandj]

Vincent,

Additionally Djandj is using some sort of plug in his mag since he is in California and mag capacity is limited to 10 rds. Since it is a 25 rd zb39 mag he would have to have a plug equivalent to 15rds so that his 10 rds are rd 25 to 15. The mag may be causing enough drag so the the bolt doesn't lock properly. Easy enough to check. Don't use a mag & see what happens.

Joe

Joe - just to eliminate the issue, the way I limit the magazine to remain CA legal is simply a wooden dowel cut out curved to fit along the back channel of the mag. It does not exert any pressure on the mag follower it just rattles around until the follower goes down to the 10th round and then stops the follower from going any further. As far as the pressure from the mag it is no different than putting only 10 rounds in a 30 round mag. (very little pressure)