I have heard of people getting themselves killed using desert eagles and 44.magnums. And honestly I am afraid to know how using that would turn out. Besides why on earth do you want a submachine gun that fires 50. cal rounds? I mean yeah the guy with this thing would feel like and maybe even look like a badass up until they pull that trigger....
The problem with this design is that the bolt is too long to recede into the down position. The section between the grip and the mag would have to be extended, and the bottom of that bit would also have to be around the magazine's length to be able to absorb the recoil and return the bolt forward without damaging anything with sheer force. The barrel's length is probably pointless for the .50 bmg as the powder wouldn't all catch in the barrel, making most of the powder explode outside of the barrel and in a big fireball. The round would possibly also tumble from lack of spin. Kriss is, however, designing a .50 Beowolf system. That's a bit more feasible and would make better use of the recoil reduction system. This still looks cool, though.
Oh, and the ejection port is exactly the size of the magazine. The magazine would be longer because the round has a bullet at the tip. So the ejector port could just be made shorter here to compensate for that.
Nice concept, but its range would be measured in feet, with single digits with a barrel that short. Add some length to it, increase the bore diameter so it looks like it can actually handle the caliber and I think you've got something.
Its for wallbanging, door-breaching and CQB (They should design a hollow-point .50 for that)
About your previous comment on the ejector port size, im not sure about that. I have never handled anything except a .22 bot-action, but there must be a way to easily extract a misfire, (a cartridge that doesn't fire, due to a damaged primer for example.)
An AK ejector port for example is waaay longer than just the empty shell.