What do you need to do with your L4A3 Bren Longshoar? If it's just to show it, then what's the matter with machining down a bog standard ten-a-penny Mk2 barrel, fitting a fluted flash eliminator. Have it b;lasted, phosphated and painted and there it is. Same as skimming the front off a bog standard .303" breech block.

If it's to fire blank at a show, just insert a 7.62mm chamber into a Mk2 barrel and then proceed as above. We used .303" barrels with new chambers inserted while I was in Australiaicon. I saw hundreds of them!