This is either a movie or an episode of a television series that I saw as a child on TV in the mid-'80s, and that I am trying to identify. I believe it's a feature film from the '60s or '70s.
The movie I'm thinking of is a thriller, and I only saw the final sequence. It depicts a building with an unusual, futuristic-looking "gun" inside - think a sci-fi type of gun attached to a detonator. In the final sequence, the gun is pointed straight down and begins rotating slowly upward toward the camera. As we see the gun rotating up toward us in slow-mo, we periodically cut to a group of frantic people - one of whom is a man with very long, stringy hair flying upward - racing to get out of this building alive, before the gun reaches a horizontal level and demolishes the structure; I believe they either take an elevator or the stairs to reach the ground floor, and just as soon as they hit the ground and go racing out of the building, the gun hits its detonation point, emits a laser, and triggers an explosion.
Does this sound familiar to anyone?