Repairing Bren Mk 1 barrel

[Flying10uk]

So what happened with the conversions to L4, 7.62mm, Peter? Did ZB have to agree/license the conversion or was the original license agreement null and void by that time?

[Peter Laidler]

Another long winded answer would be forthcoming and 4 pages on, we still haven't made progress on the hole-in-the-chamber barrels. Maybe our energies ought to be directed there.

[CINDERS]

From what you are saying Peter with the design of the Bren if say yourself made a deviation from the design that was an improvement there were penalties applied to the MoD and were field mod's under the same auspicious just curious as to the workings of design and copyright & so forth. I thought anything that made the weapon better and more efficient at its task would be a bonus. TIA

[Flying10uk]

I don't think that, in the case of, the MK2 BREN it was an improvement, CINDERS, more of a basic/utility version of the MK1 BREN.

[Peter Laidler]

If we made a production improvement or change, we paid. Modifications later were down to us. However, there were two that were discussed and taken on board by the manufacturer where we didn't pay. But on the other hand, we didn't get return royalties either

[Joe H]

Joe H, I got the mill test certificate for my barrel blanks.

Fortunately they were heat treated and have a minimum proof stress of 96,000 psi!

As others have mentioned I needed to get oversize blanks - I went with 35mm from memory.

Richard,

Getting back on track. Where were the holes drilled? Diameter? Where they on the small diameter or on the thread diameter? Just one hole?

That 96,000 psi test really doesn't tell you everything. It was done on a 35mm? blank. On the Bren much of the chamber is only 0.9" diameter. Doing a little calculation ( barrel dia = 0.9") the 65,000 psi proof test (chamber pressure) would give a maximum 108,000 psi hoop stress in the barrel with the proof load and only 80,000 with the normal load. If this is the typical barrel steel, Rc 28-33 the Yield is 125,000 psi. The proof load, one time only, is below yield. Working stress is less than 64% yield

Also, to get to those strengths the barrel was probably tempered after quenching to around 1,000 deg F. Any heating of the barrel for heat shrinking should be kept below 1,000 or you will risk losing strength in the original barrel.

Joe

[Flying10uk]

I think that a few pictures would be very helpful in giving everyone a better understanding of what state the barrel is presently in.

[Richard-]

I've attached a drawing showing what I had in mind for the barrel repair:

http://www.rarco.com.au/wp-content/u...9/20160909.pdf

The hole is 8mm diameter, drilled through one side of the shoulder of the chamber, as drawn.

I'm keen to try a plug repair, to keep the barrel as original as possible.

The idea of the reduced diameter step is give the thread shoulder something to bear on, and to keep the chamber surface as clean as possible - no divits from the thread ends.

I meant to post this with the original post - want to make sure I get the best possible solution, hence the too-and-fro.

There's actually two projects here - the second is to fabricate new barrels for an L4A4 7.62 gun, hence the barrel blank discussion.

[Peter Laidler]

I've said my piece on your proposed repair and say no more except that............ I made 2 mistakes in my suggestion to take the KISS and safe course and simply rechamber. The first was that I said it would take 2 hours to bore out the chamber and machine down the new ex rifle chamber insert. WRONG. It takes a lot less.

The second was that the old No1 rifle chamber I used was about .003" eccentric believe it or not! Yep, it's true! But it is an easy fix because the second time I tried (thanks Rxxxxxx for the scrap donor barrels) I made a taper centre for the chamber with the already known taper of the cartridge case, threaded at the neck end which exited the cut off chamber. Nut on that end, barrel stub held tight onto new centre. Centre put in chuck, barrel stub chamber now definately concentric and turned down successfully and concentric - ready to press into bored out chamber.

7.62mm barrels. Richard........, this forum is littered with peoples day-dreams of producing them....... and others day-dreams of buying and owning them. And which version of the 7.62mm barrel version of which 7.62mm gun do you want? Guess how many have succeeded so far. Call me defeatist if you like but having used, repaired and worked on them with a megga deep understanding of the design and build, I say that UNLESS you stick to or retain the exact exterior profile and alignment of an original known barrel, it ain't goin' nowhere. Just my view

[Joe H]

Guys,

Here is my 2 cents on the proposed plug method. The plug must resist two types of forces. The first is the chamber pressure which is resisted by shear on the threads. The force is easily calculated by the chamber pressure on the area of a 10mm diameter plug. The threads must be perfect for the plug shear resistance area to equal the plug circumference (at root of threads) x the length of the threads. The second force is the hoop tension caused by the chamber pressure. A threaded plug offers no resistance to this force because it is circumferential tension. It's like the plug was not there. Stress-wise what will happen around that plug probably can only be determined by some sort of computer stress analysis. There will be some sort of stress riser around the plug area.

For you guys to kick around let me offer something similar to what Peter proposed. Make the sleeve out of something like pre-hardened 4140. Say around 32 - 33 Rc , Yield around 130-135,000 psi. Even at Rc 28 Yield would be around 120,000 psi. I have used this material and it is easily machined with HSS tooling. Now if we make the sleeve approx. 0.9" dia (rear dia of Bren Barrels) and totally remove the rear .9"dia of the existing barrel, bore for a shrink fit into the threaded portion of the barrel and make the sleeve long enough to cover the hole then shrink fit into the 1.26" diameter section of the barrel, the sleeve is totally capable of handling the chamber pressure on its own and the hole can be left as is or a pin riveted in for appearance. Machine the end to match the Bren barrel and then ream the chamber.

7.62 barrel thoughts in another post

Joe