Okay, I'm currently looking for the tube, but here's what I'm planning.
Basically, I want to take an M72 tube and convert it to fire 40mm grenades, probably the stryofoam rocket variety for anti-vehicle use. Just a push-button system activating the grenade. BUT. The system will activate TWO grenades. The first will be the normal, forward firing one. The second will always be a canister round of a certain size (to be determined) firing backwards, to create a backblast cone. This will force realistic use of it, with the operator having to check the blast area first. Thoughts?