The Gauntlet (1977)

Phoenix cop Ben Shockley is well on his way to becoming a derelict when he is assigned to transport a witness named Gus Mally from Vegas. Mally turns out to be a belligerent prostitute with mob ties and incriminating information regarding a high-placed figure. Shockley's suspicions are aroused and the betting line against Mally testifying begins to climb when the transport vehicle is bombed and Mally's house is literally shot to pieces. The pair must face the truth about those they trust, as well as their own inner strengths, as they fight their way to Phoenix, finally using an armored bus to run a gauntlet of hundreds of police armed with high-powered weaponry.