54R Bren Issues

[djandj]

Joe - THANK YOU for your help (and patience with me). If that is the EASY way to check headspacing, I'd hate to see the hard way. That operation just assumes I have .0003 shim stock and a feeler gauge just laying around. (as I said, I'm not a metal worker or gunsmith.) I understand headspacing as it applies to my 1919a4 for example. The distance from the end of the barrel to the bolt face. However, when the barrel has huge cut outs on it (exactly where the cases are blowing out) I don't see you can ever make the headspacing tight enough to matter.) Again, I don't know. I followed your directions as best I could and took pics. Not sure it helps. (see attached) I suspect that I can send it to someone to get the headspacing checked.

Question: what are the "locking shoulders" everyone keeps talking about? And what is movable in the receiver to change the headspacing anyway?
Last observation, your second hypothesis (i.e. that the gun is out of timing- makes logical sense as it does appear that the bolt may be moving back before the pressure has dropped to a safe level.) Can this happen only once after 60 rounds or so? (B/C that is when it happens). What can we do if the rifle is mistimed?

Again, thanks for all the help. I very much appreciate it.

[djandj]

OK Disregard the locking shoulders question. Figured out what they are. I certainly don't appear to have the gap Joe was talking about and showed in his pic. Of course I don't have a feeler gauge either.

[tankhunter]

Feeler Gauges should be readily available from Auto tools & Accessories stores. Also good hand tool retail & trade outlets.

[Peter Laidler]

If I can just chip-in here for a second......... The FIRST thing you MUST do is get down to Brian at BDL and get the little SAS booklet about the Bren. It's important that we are all on the same wave-length when it comes to the names and identity of parts - such as the locking shoulders.

The SECOND thing you ought to ask is this. What EXACTLY is the CHS spec for soviet 7.62 x54R. I was resonsible for certifying that the regularly used foreign weapons were '....certified fit to fire by a current and competent authority' - me! Trying to find out the bore gauge size and CHS was virtually impossible as it had to be the weapon or ammo manufacturers specification. And manufacturers spec isn't the SAAMI or proof house or uncle Tom Cobbley or some of the figures bandied about or........ You get my drift. So I used to write out a well tried and tested format while certifying a weapon fit to fire. So that's the first tying you need to know - the certified CHS.

As for what you call 'the timing' during the unlocking phase. From the pictures and description it's not clear whether the unlocking system in your gun is still by means of the unlocking cam part of the piston post acting to draw down the rear of the breech block. If it is, then the timing cannot (?) be out. This is because - and I'll keep it very technically simple - the dynamic and engineering geometry is such that once the bullet has passed the gas port, the gas MUST impart sufficient energy to move the heavy piston, piston extension, piston post etc etc (the PPE) .5" just to reach the point at which COMMENCEMENT TO UNLOCK starts. In simple mathematical terms, by the time the PPE has moved this .5", the bullet has moved 30" - or 2'6" and it's well clear of the barrel. But there's more..........

There is some discussion as to exactly when the gas pressure in the barrel returns to neutral/atmospheric. Some say it's 6" after the bullet has left the bore. Others, like me say that at 1" (or 3" if the flash eliminator is fitted) the gas pressure is at a safe working load. So given that the PPE has to move at least 1" to fully open/unlock the breecch block, during which time the bullet will have moved 60" past the gas port there is an inbuilt and infallible mechanical safety during your guns unlocking phase.

[Joe H]

Peter,

Regarding your comments on "timing". You have the experience with the Full Auto Bren of which I have none. The striker semi auto version, which djandj has operates somewhat differently. In the full auto Bren the return spring drives the full mass of the carrier, piston and bolt forward to close the bolt and fire the round. In the semi version the bolt must be closed separately then the striker is released by the trigger to fire the round. The same return spring must now move the bolt, carrier and striker(additional mass) forward and cock the added striker spring. The stroke is now shorter due to the presence of the striker behind the bolt and carrier. The return spring cannot be compressed to its original design compression. This can be alleviated by adding a plug equal to the length of the striker behind the return spring. I don't known if this was done in djanj's Bren.

My own observations in building a semi Bren are that due to welding the receiver together and the spring issues mentioned above it is possible that the carrier does not travel freely and does not return to the position indicated in the pic I posted above allowing a shorter or no travel before the bolt unlocks. The lower part of the striker in a semi normally is the same x-section as the carrier and runs in the same grooves behind the carrier. A tab extends above the lower section which strikes the firing pin. The upper tab is many times flush with the front of the striker allowing the rifle to be fired if the carrier is short of the correct "in battery" position. If the tab is located 3/8" behind the front of the lower part of the striker then if the carrier is not fully forward the lower striker will hit the carrier and not fire. I don't know if this safety feature is built into djandj's bren. Most of the semi's I have seen do not have this feature.

I couldn't find any headspace data on the 7.62x54r. Pressure would not be an issue with Bren but I'm not sure how relevant a "Field" gage measure would be since it was never meant to be used in the Bren barrel. The conversion has been successfully used by USA semi Bren builders. It works in my Bren which has very tight headspace. You can see from the pic below which I previously posted that there is little margin for error due to the depth of the solid base of the 7.62x 54 rounds. The .303 round has a much deeper base and consequently more tolerant of loose headspace.

IMO this why some people experience no problems with .303 British in their semis but start having problems with the 7.62x54R.

Joe

[Peter Laidler]

I'm going to the US next month or so, so will arrange with Brian that I can see one. I appreciate that it is difficult for me here even picturing the mechanics of a semi auto Bren there. But it's a credit to the semi-designers for their work. Me, I'd just stick to .303"

[djandj]

OK Guys. Feeler gauges ordered. I will try to get an accurate measure of the headspacing so we can start there. After that, it has been hypothesized by Troy at Barrel Works that since the gun tends to go BOOM on the first round of the mag each time that it could not be fully locked up when it fires. (IDK) He had suggested looking at the extractor and making sure that it wasn't getting in the way of a solid lock up. Who knows? But head spacing measure first. I will then report back.

Thanks again all.

[Brian Dick]

I don't have any semi-auto Bren guns here. My two guns are both restricted, post sample originals. There may be some to look at in PA. Wiselite Arms was there last time, (set up directly behind me in fact), which would be nice because they did a much better job than Historical Arms IMHO. They had them in 7.62x54R too if memory serves. I don't know if they're still building them or not since that was five years ago.

[WallyG.]

Brian:

If you wish... I can send one of mine for you to test drive while your entertaining guests... just let me know.

ATB

Gery Hackney

[Vincent]

Brian, I agree that Wise Lite Arms did a better job. Their semi setup is different and better IMO. But the big difference is the quality of their receivers.

They stopped selling Brens a while back, but they still have a receiver weld service and a repair service.