Can i rechamber my interarms .375 to shoot 416 rigby? Does anyone know a rough cost to do this?

Although it has been done, the standard length Mauser like your Mark X is really too short for the 416 Rigby. You might be able to make it work with the 416 Remington. You'll have to rebarrel (about $300 up). I don't think there's enough meat on the barrel to safely rebore the factory 375 barrel to 416 (which would also cost a minimum of $300). If you want a Rigby at a relatively low cost, sell your Interarms and apply the proceeds toward a Brno ZKK or CZ 550 (around $600-$700).

fla3006 is correct that selling your Mark X and buying a CZ 416 Rigby would be the way to go. If the conversion is possible on your Mark X, it still costs you at least several hundred bucks for the project so you might as well aquire something new designed for the cartridge. I paid about $625 for my CZ 550 Safari in 416 Rigby and it is one heck of shooter out of the box. Good luck.

greg p'
I'll give you $15.00 for it. I'm sure that 300grainers will just probably bounce off of blackbears, but I'm willing to sacrifice so you can purchase a 222 Rigby!
Are you back?
best,
bhtr

greg p,
Call me on the "land line" I'm up till midnight.
There are bears prowling this weekend yet!
best,
mark