He is holding a Thompson sub-machine gun, AKA the "Tommy Gun."
This little humdinger carried a 20 or 30 round magazine and/or a 50 or 100 round drum in .45 caliber - quite a heavy pistol bullet.
Just forward of the pistol grip, you can see the trigger well, and just forward of that, you can see the space where the magazine or drum goes - in this photo, the submachinegun is completely unloaded as you can see in this photo: