Game designers shouldn't design guns. This is the most retarded thing I have ever seen.
So the magazine is well out in front of the trigger like an AR, but that would mean the blow back of the slide would have to travel backwards at least the total width of the magaine, the magazine looks to be about 1.5 times the width of the grip, so maybe 2-2.5 inches?
So a 500 SW, overall length of 2.3 inches would work, or 454 Casull - 1.8 inches total length, or 50 AE (Deagle - 1.6 inches long, but why wouldn't you put the rounds in the grip?)
I don't know what that "shroud" is for over the barrel, but the reduced sight radius irks me to no end.
It's not ported, so recoil/muzzle flip is going to be a bear, quick follow up shots are not going to happen. The average sized guy is not shooting this thing one handed.
The pistol just looks fucking unwieldly. Look at all that mass forward of the trigger. That's going to suck to hold steady, even with two hands for any length of time.