Epic is adding a Kalashnikov-styled Heavy Assault Rifle to Fortnite, and it'll be "most useful when fired in short bursts".
The gun, above, appeared yesterday in the updates feed that opens when you launch the game. It's described as a "powerful assault rifle" that is "coming soon"—usually, that means it'll be in the battle royale game within a week.
The fact it's a "heavy" assault rifle probably means it'll deal more damage than the standard M4. It'll probably be less accurate, hence it being more useful when fired in bursts: I'd expect a fairly big recoil spray if you fire it full-auto.
For the latest on what might be hidden inside Fortnite's mysterious cube, which is about to pop, read James's piece from yesterday.