Our ideas are what we call '.......same meat, just different gravy' Joe. The only reason I say use an old rifle chamber is that it is already a known quality and dimensionally correct and therefore cheaper and on the face of it, simpler of course because all the metallurgy and chamber cutting is already done for you. Just my take on K.I.S.S. of course. On the other hand you could use a shot-out Bren barrel stub to achieve the same result. The good bit is that if it should fail, then you just pull the insert out and start again.
The theory of both methods is absolutely sound as our original FN-MAG58's L7/GPMG's came with chamber inserts*. Not only that, even today, the enlarged section at the rear...., the bit that holds the barrel locking nut and carrying handle is a pressed on part and pinned
Bren and rifle barrels are a pleasure to machine with good cutting tools but I feel this idea of plugging as in the drawings shows a degree of, er......., what is the most diplomatic way I can put this......? Anyway
*As mentioned earlier, it failed for OTHER reasons and not through gas leaching or the insert moving
Have a nice rest of the day Joe from Abingdon in Oxfordshire