When you drag shotgun ammo over an MP-133 Shotgun with pistol grip (no matter if on ground, back or in hand) and choose "Chamber", the game will chamber the shotgun with four shells. If you now shoot, you have to press R and it should cycle the old shell against a new one. Instead you fire the shotgun, press R and the full shell will appear in your inventory as if you never fired it. Like that, you can empty your internal magazine by firing and hitting R after each shot, just combining the shotgun rounds again and then load them again, effectively giving you unlimited ammo.