I suspect that the gun is made of materials that cannot stand up to real world ammo, and that the barrel is well, I was going to say obstructed, but that implies that there was once a real barrel, and that the blank it is chambered in is short enough no round (save maybe a CB cap) would physically fit.
Now, sure, you might well be able to convert it to fire a single 22LR, but then, making a zip gun from scratch would probably be easier.