7.92 chamber depth

[Peter Laidler]

Just a bit puzzled how you can bring the barrel rearwards by .1" into the body Wally. It's simplicity itself to machine the body seating face of the barrel back by .10" or so (but you'll be bringing the gas block very close to the front face of the gas cylinder) and this will indeed bring the barrel rearwards. The problem as I see it now is that even if you use the largest number barrel locking nut (the BLN) it will still not take up the 'lost' .1" needed to take up the slack in order to bring the new body seating face of the barrel tight up against the barrel seating face in the body.

Changing the BLN's is what we do of course to take up wear in the barrel and body seating face due to wear and tear - and fit new barrels of course..... Semi or re-weld guns might work with totally different mechanical design geometry of course

The irony of this calibre change project is that even Enfield, who had all of thge original design and build spec encountered ongoing problem after problem finalising the ultimate L4A2 programme. One exasperated now retired designer said to me:
Designer involved in conversion project to Author
........one designer involved in the project rolled his eyes and looked skywards in desperation and exclaimed “…we used and ruined a LOT of Bren bodies…, it caused us nightmares. We were working from drawings and the problems were not immediately apparent. The actual mechanics of it were only visible when we sectioned an actual body. The answer when it came was simple,

Here are extracts of some notes I made while writing up at work when I had access to more info, parts and guns!
As a point of identification, note that British 7.62mm (and Canadian 7.92mm) breech blocks are 4.52” long. This measurement is taken from across the front edge of the breech block, over the firing pin hole to the rearmost point of the breech block. (The .303” block being 4.485” long) and the 7.62mm/7.92mm type locking shoulders vary in length between .777” and .790” long (the .303” versions being between .819 and .832” long). These measurements are taken from some 30 assorted breech blocks and considerably more locking shoulders.

One last point while I'm in Bren mode......... Sorry to ramble on a bit, but I can see problems ahead in using a .303 gun breech block (the BB) in a 7.62 or 7.92 gun. When the cartridge is in the chamber ready to be fired, it will be, by definition, aligned exactly with the centre line of the breech block/striker and as such, sat exactly central in the larger dia cartridge seating face (the CSF). All well so far....... Now the gun has fired and cart case is being extracted from the chamber. But NOW, as the two tapers (cart and chamber.....) are being drawn apart, the spent case is not held tight and secure within and hard against the larger CSF in the BB face. It is now possible for the spent case to move away, upwards (.025"?) from the extractor claw and out of the control of the claw and BB.

This feature where the spent case is held hard, firm and secure to the CSF in the BB is critical and a very important part of the extractor stay and tapered extractor stay ramp in the piston extension.

Maybe I am complicating something that doesn't need complicating or that absolute reliability in a service gun is more important than the odd jamb or mis-fire on a civillian rifle range