Arlen Roth was Macchio's guitar tutor for the film and played many of his pieces in the movie. According to Roth on his site, arlenroth.com, the final duel was arranged from parts played by himself, Vai, Cooder and Bill Kanegiser, who did the classical playing throughout the film. Roth wasn't credited for his work on the movie, due to a contractual disagreement.
At just about an hour into the film, in Crupps bar, the bartender pulls a shotgun to break up the fight. He racks the pump, because that is a sound movie audiences associate with a shotgun. The only problem is that the bartender is holding a double barrel over-under shotgun that opens up at the back, it is not a pump. And it will not make that sound.