Hey guys, nice one Kev on the info here, I can report 100% success with a marble inserted in AIS pipe.
No more backfiring now at all, a little disappointed I can't frighten old ladies now.
On the other hand it was embarrasing that post 11pm people would open their curtains to see who'd been shot.
I did choose a marble that was ever so slightly oversize of the pipe and eased it in with a tiny bit of silicone lubricunt grease.