300 grain bullet at 860 fps
Although the .50 GI has a 300 grain, .50 caliber bullet traveling at 860 fps, you would need a dedicated gun to fire it, and I'm not sure of the bullet designs available for the round. I think that bullet design may be equally important to mass and velocity in determining performance of the ammunition.
As an alternative, most modern .45 autos (which most of us already own) will handle .45 +P ammunition, made by several major manufacturers in various bullet designs. If our standard .45 ammo is not hot enough, we have the option of upgrading to a +P selection.
For example, Speer sells a 200 grain .45 +P Gold Dot that has muzzle velocity of 1080 fps and muzzle energy of 518 foot pounds. That is pretty good for a standard .45 caliber gun, and I would guess that it might equal the .50 GI in stopping power.
Speer 200 grain .45 +P Gold Dot