I saw this one on HBO as a kid in the 1980s. As best as I can remember, the plot was along these lines: There's a series of murders occurring, and the protagonist, I believe, is a suspect at one point. IIRC, he's at or near the scene of one of the murders. The protagonist can't remember what happened during the time of the murder - what he was doing, etc.
As he investigates, the protagonist discovers an experimental weapon has been developed. It's a handgun that fires pulses of light. The pattern of the light causes the victim to go in to a sort of blackout. Not really unconscious, but sort of stunned in place. When it wears off, the victim is left with a memory gap, with no recollection of what happened or what took place while he was stunned.
I recall the protagonist is able to acquire the weapon, and turns the table on the man who's been killing people with it. I don't recall how the weapon was developed, or why the man was killing people in the first place. I do remember the protagonist beats the killer up by using the weapon to stun him and then punching him out between bursts.
I saw the film in the early 80s, so I'd say 70's or 80s. It was a color film, and I remember it being fairly "modern"-looking - i.e. not an old 60's film or the like. That's all I can recall.