Who knows what's the name of the episode..its about a commercial maker or a film maker
He catch the killer with seeing a commercial on a tape. On that tape you see the wappon or something
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.