I know the .357 can be changed into a .44, but can the .50 be changed into all of them? I would like a .357 auto, but the coonan is kinda hard to find etc.  And if I am going to spend that much I might as well go all the way.

I don't think the Mark I was convertible.  

On the Mark VII, you could get kits to convert between .357 & .44, or .44 & .50AE.  I don't think you could go from .50AE to .357 on the Mark VII.  The Mark VII has been discontinued, but new ones are still out there.

On the Mark XIX (the current model), you can switch between 357, .44. .440 Cor-Bon, .50AE.  You can (or could) switch between .50AE and .440 Cor-Bon by just changing barrels.  (I said "or could" because they're no longer listing the .440 Cor-Bon on the magnum Research website.)  To change to .44, you'll also need a different magazine.  For .357, you'll need a differnt bolt assembly too.

So make sure you get the Mark XIX.

ETA: Here's how to tell the difference:



More info here.

Thanks a lot john

for the mk xix you can buy the whole smash and convert between all 3 calibers--.357mag, .44mag, .50ae.  iirc no tools are required.  and yes you need different mags for all of them.  and i think you can buy the .357mag version and other barrels etc later.