Repairing Bren Mk 1 barrel

[Joe H]

Guys,

I got interested in this thread because as I have recently posted I have a L4A3 parts kit without a barrel and since it becomes increasingly possible as time passes that Sarco isn't going to be able to supply a US made L4 barrel. If this comes to pass I have a 7.62 barrel blank on to which I'm going heat shrink the interrupted threads from a Bren barrel on the barrel blank and then turn the blank to L4 dimensions.

That said, in any Bren barrel the rear section of the Bren barrel around the chamber is only .9" in Diameter. The blank I have is Rc 28-33, very similar to Dutch spec consequently the Yield is around 90,000psi. and the barrel blank has been heat treated.

Here is info I found on the various calibers. Proof on .303 estimated at 33% above normal.

Max chamber psi , Proof psi

.303 British 49,000 , 65,000

7.61x51 Nato 50,000 , 67,500

.308 Winchester 62,000 , 83,000

Considering the chamber as a thick Walled cylinder:

Normal Stress Chamber walls psi, Proof Stress Chamber Walls psi

.303 British 47,300 , 62,850

7.61x51 Nato 48,400 , 65,300

.308 Winchester 59,900 , 80,250


All the proof loads are above the Yield of the un-heat treated steel (62,500 psi). The normal .303 & 7.62 have a factor of safety between 1.29 & 1.31 using untreated 4140 type steel. The max loads on the .308 have virtually no factor of safety using annealed 4140.

All stresses are below the 90,000 psi yield of the heat treated 4140. I guess this tells why why the barrels are heat treated. It also shows the items to be considered when dealing with modifying a barrel.


Joe