I would have to say Barrett 50 cal.
That's because they are different cartridges... A .22 is a rimfire rifle cartridge, the shotgun fires shotgun shells.the bullet doesnt tell how powerful it is all the time. it is just like a shotgun is more powerful than a 22 no matter what kind of bullet is in the shotgun!!!!
The higher the grain (weight) of the bullet, generally the more powerful, so yes a 800 grain 50 bmg will be more powerful than a 647 grain 50 bmg. It also depends on the length and how 'hot' the cartridge is loaded.It could be like that for sniper rifles too one is more powerful