Lancaster, I wonder if the reason the 8x50R is the intended round for the Sporter you mention is that such a caliber would clean up existing .303 chambers/barrels nicely AND it would eliminate the use of military ammo.

A rebore in 8mm would/should allow a smooth finish and eliminate all traces of the original boring, assuming the .303 barrel wasn't grossly overbored in the first place, a 8x50R chambering reamer would clean up the .303 chamber AND probably the round would feed fairly well {??} from unmodified .303 mags.

The military ammo issue might still be a problem as much military .303 has been made in recent years in India, and may still be.

Anyway, it's an idea.