Found a bug in multiplayer that occurs when you have both the suppressor and single shot attachments on a gun (occurs when using the Grendel); when not using single shot, the gun shoots silently, but when you switch to single shot, the gun shoots normally as though there is no suppressor.

Like I said, this may be an issue only with the Grendel as it's the only gun I've tried using with this combo, but it's likely the same with all the guns.