Frank,

I'd stick with .311 as that is the proper diameter. (I'm not even sure if you could get a .312 aside from a custom job.) Most of the bullets are either .310 or .311. If you neck-size the brass only you should get good case life.

There is an excellent website on everything .303 run by an ex-armourer in Ontario, Steve Redgwell: www.303british.com. Steve is also making 200 grain bullets for the .303, and in .311, .313 &.314 diameters (to suit military tolerances and worn bores).

You might also consider having it chambered for .303 Epps, which is like an Ackley improved- it gives a little more velocity and better case life. I've had my BSA-sporterized P-14 rechambered for .303 Epps and am looking forward to working up loads for it.

Stuart