"Are these conversions limited to certain calibres or are they open to all and sundry? What cals would they be limited to if this is the case?"

Pressure is the biggest consideration when building double rifles, especially on shotgun actions. The size or power of the cartridge is not the real issue, pressure is. I just don't like anything that is over 40K-45K PSI - my personal restriction. Vic Venters in his great book "Gun Craft" discusses fitting "to the circle". This is the most concise discussion that I have ever read on fitting barrels to the action, and this fitting is crucial with anything in the higher pressure ranges (40K-45K PSI). Frequently, shotgun barrels are not fitted tightly enough to the circle to allow the higher pressure stuff, but they work just fine for the old British, low pressure, nitro express rounds. Aside from physically fitting on the action, bigger is not a problem, but pressure is. I just finished a .470 on 12B frame - was great. I have not built a .500NE, but feel it is the biggest I would do on a 12B frame, mostly because the barrels are going to be lighter because of the bigger hole and therefore, it will be harder to get the weight up where it should be for a .500NE

The only issue with anything bigger than a .500NE (and I am not talking 505 Gibbs or .500 Jeffery - those are bolt gun cartridges and should stay there) is the action size. Anything over a .500 NE and you really need suitable 10 gauge action.

Ellis