This is Bristlemouth 3K (Steyr ACR), a tactical rifle that can be switched from 3-round burst fire to semi-auto rounds. It comes with a scope or not.

It should appear in Season 2. It can be obtained at Tier 15 as a free weapon from the Battle Pass. In Zombies, it can be Packed-a-Punched to Fangtooth Firing Blade. When upgraded, it will randomly fire explosive rounds.