I think the idea was to fire a pencil using a blank cartridge, not a ball round! Blank 9mm ammo could be obtained from captured German soldiers on exercises and was much prized as it could give those not in the know a nasty fright!
The idea was to use no ammo at all, the fact that to fire off a Brownings action you needed a mag in the well didn't help, nor the fact that he cocked the pistol with the loaded mag in ,total moron.