It's probably a bug then. I also got the visual glitch with the Tac Laser on, but not visible on my side. I believe it's always visible to opponents though. Lasers also seem to disappear whenever you aim down your sights.

One thing that may also affect this challenge: The laser is not always visible. Perhaps you need a laser visible to enemies, and to hip fire to get the kills. Have tried with one of the two laser attachments that are visible to the enemy ?