I don't want to imagine a .50 BMG snub nose. I am afraid if I do, someone will make it, and in Scandium to boot. Then people will want to know what grips to buy to reduce recoil, what porting works best, etc., etc.
For a rule of thumb, add/subtract 50 fps for every inch of barrel. IMHO, .38 Special is quite enough in most snubbies.
As someone mentioned on another forum, the gun shops are full of slightly used .357 S&W snubby lightweights, with a box of 47 rounds included.