SB:
I know how other variants of the trick are done, as you describe. Unfortunately for explainations, I signed both the bullet and the shell, ensured that it was indeed placed in Teller's gun, nobody crossed the dividing line, shots fired, I took the signed bullet out of Penn's mouth, and verified rifling marks & powder burns on the bullet and a distinct smell & appropriate discoloration on the shell (which I removed from Teller's gun). Everything indicates a legitimate firing. Beats the heck out of me!
Long Path:
I'm quite aware that some magicians plant assistants in the audience. I have conclusively proven to myself that the audience members chosen to verify the trick are not "plants". I know that doesn't satisfy _you_, but that's your problem
. If you're still skeptical, I recommend you go to the show (~$35), get near-front-row-center (I was at fourth row, slightly left) seats, and volunteer (be ready to give a very brief explaination of how you know guns; "trained at Gunsite and Lethal Force Institute" worked for me). Some things ya just gotta prove to yourself thru experience.
George:
There were two glass plates. One was pretty well destroyed, but the other had 1/3 of the distinctive "shot glass" crack pattern. The bullet in Penn's mouth was indeed the one I put in Teller's gun, and had rifling marks and powder burns and smelled freshly fired.
As for discomfort, the first time I saw it done live (last year), Penn did indeed look uncomfortable while he seemed to be moving the bullet to his teeth.
The shots did not seem as loud as a normal .357; the rounds may have been under-powered, loaded just enough to cross ~10 yards, penetrate two panes of glass and land in one's mouth without harm. (Heck, that's the best explaination I can come up with!)