It sounds, icecold, as though you still believe that there is a real bullet in the gun - I very much doubt it. The shattering of the glass may be synchronised with the firing of the gun, which contains a blank, the marked bullet being swapped at some stage. A trap people fall into is assuming that there is just one secret factor, which if revealed would explain the illusion. But top-class illusionists employ a complex mix of mechanical devices, manual dexterity, psychology and confidence trickery. And, of course, they practice and plan their major illusions for many months, to get every element right.