A much safer way to get a "down the barrel" shot of a discharging weapon is to fire it at a mirror.

Angles of weapon and camera give the appearance of looking down the barrel. Actual discharge is in a safe direction, and the mirror itself may be backed by energy-aborbing material (sandbags, gel, etc.) The resulting imagery is of course quite striking.

I've looked for but cannot find any examples readily. I was thinking that either Hal Edgerton or some ultra-slow-motion footage used this method.