The front section of a 303 chamber is basically the same as a 7.62x39, you don't have to re-chamber, just shorten and re-thread.
I've used old smelly barrels to convert Carcano's and early Mausers, 93's and 95's, to this calibre.
The mag conversion is a bit trickier, depending on the action used, but single row mags simplify things a bit.

Roger

PM if you need more info.