I thought long and hard about this when I read it. On the plus side, we did exactly the same with our early heavy barrel L7/GPMG's with a stellite chamber and 4"(?) length of barrel ahead of it all pressed into the barrel. The riflingh between the chamber insert and the actual barrel didn't even mate-up. There were problems but nothing that would relate to a rifle.

What'd be the problem with the new chamber and leed being threaded for, say, 1/2" at the front end of the insert and similarly at the front end of the now bored out barrel chamber. Screw in place, lock and make good. No rifling to mate up (not that it is necessary of course.....), just screw in tight, lock and bobs your uncle. I'd say not a lot of work if done systematically, carefully and accurately.

I don't think that an insert per-se is a problem as we saw with the GPMG.

I have never done this except to lock in place a .22" LR chamber insert into a sub calibre SA80 during early trials to test the feasibility of certain rifles remaining .22LR. During that trial there was no question of gas leeching behind the insert. So feel free to shoot me down.

Could this be the answer to convert .303 Brens to 7.62x54R?