Barrel could be threaded for any of several possible attachments. In Europe, some countries allow these blank guns and the carrying there of that don't allow "live" weapons. Thus attachments exist - variously - to expel tear gas and another option is to expel a rubber ball with enough velocity to theoretically stun someone/give them an owie. Other blank guns have been designed to launch small objects as an aid in dog training, small flares, etc.

Coibel made James Bond cap guns on the same lines/principle at one point, possibly off a mold of one of the blank guns. These are still made, so I suppose that you might be able to get a new "magazine" for it.