When I was in Australiaicon we had a small programme whereby a round block containing a 7.62mm chamber was pressed into the bored out rear end of a standard .303" barrel so that it became a BFA barrel for the 7.62mm L4's. You COULD try something similar but using a .303" chamber machined down from a scrap No1 rifle barrel. That'd be simplicity itself. But not something I'd bother with..........

Making a new barrel.......... Not something I'd undertake lightly. Far too much a) machining and b) accurate geometrical features involved