if i were to do testing of this sort, i'd invest in a n inexpensive 12 gauge single barrel, like an H&R. you can get these for 80 dollars at your local pawn shop. the sides of the reciever are flat, so you can chuck it up in a vise easily.
i'd chuck it up on a vise, bolted to your shooting table, tie a string to the trigger, and fire it that way.
you'll not have to Worry about blowing your face off, then.