Got playing w/ the 303 Britishicon SVT concept some:

The smaller rim means the round is presented at a lower angle to the chamber- bullet noses crash the end of the barrel.

SVT-40 magazine feed lips are a bit long as the shoulder on the 303 is further fwd. SVT-38 feed lips are better suited for the conversion, IF stock mags are to be used.

Might try an aftermarket SVT mag for "butchery" purposes.
However, I'm thinking BREN mags might be the best option. Might shorten a few to 10 rds or so if it pans out, though.