I don't see why not. I was comparing the two rounds. Both show bullet diameter as being .312", max pressure of the 7.62x39 is around 51000 psi, the Brit is around 49,000. The case head diameter ( not rim diameter) is about .447" on the x39 and .460" on the Brit, so I believe the 303 reamer would clean up all of the x39 chamber. I would think that after the re-chamber, you should only need the extractor re-cut for the slightly larger diameter case head and rim of the 303. That sounds like an interesting project. It would be great if you could get the chamber cut with a reamer that did not cut the throat at the same time. That way, you could have the throat cut separately and not so long so you would get real good accuracy from all range of bullet weights and not just from the longest bullets. What single shot are you thinking of re-chambering? RJ