Hunter was exactly right. There wasn't a live round in the chamber (it was a revolver) but there was a bullet lodged in the barrel that the primer alone didn't eject from the day before. The blank was more than enough though.
I'm not suggesting that this is the proper route to accomplish this, however we've used this with great results.
sheet metal backing with foam on actor's side - firecracker taped to it with foam around it. Lit with a model rocket igniter and a small battery carried by actor.
We've been really stupid in the past. I haven't used this technique for a few years aside from in the guerilla days. Plexiglass in front of the camera protecting the DP and away we went. Had some spectactular results and were as careful as possible. There's never a foolproof way though when fire or charges are involved. Accidents can always happen.
I can safely say that the method Mike details above is a really stupid and dangerous way to fake a squib. However, I can also say that I've used this method several times personally with willing actors and it got the job done quite well!
Of course, I jsut used cardboard...so he's really taking the extra step for safety by using sheetmetal!
I have had good luck with a pneumatic solution that I built out of industrial control parts. I've included a link to my online tutorial in my signature.
It operates at a much higher pressure than the garden-sprayer techniques, and it's far more compact.