sk8ter, i know what you are talking about. i noticed the same thing. a lot of what you explained is superfluous. what i can narrow it down to is this:
purchase box pull, receive ray gun. dont fire the ray gun until start of next round. the first ray gun blast will be extra-powerful, shaking the floor/screen.
i recently noticed this phenomena but never did what you did - which is, cease firing the ray gun until the next round, re-fire it once and get the mega-blast again.
i think it might just be related to the energy build up of a full clip unused raygun that the game creators were trying to portray.