We (stupidly, of course) did squibs for our shots and they worked fairly well... ahem... minus the actual "safety" thing...
What we did:
1. Using a hockey vest with thick padding, taped two cherry-bombs in place on stuntman's chest. I wouldn't recommend this method for others, but if you do, PADDING + Earplugs.
2. Attach electric igniter (from local hobby shop) to fuse and run a wire down the back
3. Score the shirt where the blasts occur and place over vest...
4. Attached wire to a switch and a battery
5. Flip switch, wait a second, blast goes off, stuntman reacts (it's pretty much method)
If you keep the shot close in to the chest so you can't see the face, the stuntman can wear head protection!! Of course, keep in mind this is EXTREMELY dangerous, we WEREN'T professionals (just lucky, I guess) and that we did EXTENSIVE testing before ANY stunt on OURSELVES.
Realistically, you can achieve the SAME effect if the actor getting hit is really good at "selling it" and the shot is place right. Other options: air compression, shooting actor with paintball gun (again, with the actor-abusing suggestions!!), throwing condoms filled with fake blood, etc...
right. Second thought, don't listen to me if you want to live.