Personally I see the .50 bmg cartridge as a specialized piece of kit and not something that should just be cobbled together out of 2 different rifles. If you are going to spend that kind of money you should have a dedicated rifle in the one caliber. One the practical side in Afganistan Canadian snipers carried a .50 cal rifle AND a C8 (M4) Carbine with all the rest of their kit. If the balloon ever goes up, you want your primary defensive rifle available to you at all times. If you choose to convert it from, .22 LR to .223 or .223 to .50bmg, you might just get caught with the firearm assembled in the wrong configuration when you need it most.
Since a .50 cal rifle is a little on the weighty side, if I felt the need for an extreme long range rifle I think that I'd go to something in .338.
Since a .50 cal rifle is a little on the weighty side, if I felt the need for an extreme long range rifle I think that I'd go to something in .338.