Maybe it could also be PLAYBACK, except that it's a piece of evidence that's caught on tape instead of a weapon. And the character isn't actually a film maker, but he does use videotape a lot. So except for those differences, that might be the one.

I agree with DOUBLE EXPOSURE.

Columbo shows the killer a movie where he has spliced in photos from his office where the weapon is hidden. (It's actually a calibration converter; Columbo had already searched the office for the gun).