MostlyHarmless

I recently bought a 4-pack of these things and distributed three of them to some friends. For the one I kept, I drilled out the AR and added a brass barrel insert, according to Assassin's original instructions.

It worked great for a couple of days, until I shot it and the barrel cap flew off, having broke one of the plastic flanges holding it in. It seems the force of the plunger slamming into the front of the gun was too much for it to handle, and after too much stress the tab just snapped off. Whenever I fire it in this state, the barrel cap comes shooting off, the barrel jumps partly out of the gun body, and the dart only goes a few feet. I tried holding the cap on with superglue, model cement, and 5-minute epoxy, but it still came off after a few shots each time, eventually taking the other tab with it.

Right now I have it held together with duck tape, which works all right except that after each shot you usually have to push the barrel cap back into place, which is pretty annoying and time-consuming in the heat of 'battle' (not to mention it looks ugly as hell). Besides that, it works just fine with great ranges. Has anyone else run into this problem? Any ideas for fixing it?