Bolt is the same size as a Mauser's. Receiver ring is actually a bit smaller. The only reason a P14 might considered suitable for a big cartridge build is that the action is big and heavy so one might want to justify the weight.
I've actually heard some claim the P14is a good choice for a magnum because you don't have to open up the bolt face. True enough but you do have to spend hours opening up the magrecess and building a new mag box. Or you could start with a 17 and spend fifteen minutes opening up the bolt face.
I think the P14 is a great choice for a 303 British or 30/40 Krag. Life's too short to waste time on any radical alteration to end up with something which might not work all that well anyway.
I was recently sent a P14 which was a failed attempt at a conversion to 416 Rigby. It took me six hours of work to get it to where it would feed acceptably and two more to pare off some of the ugly. You can buy an action suitable for a big cartridge and build on that for less than a P14 build would cost and the result would probably be quite a bit better too. GD
