According to the official patch notes, the minigun is found in both Chests and Supply Drops (not just lying around in the world unless somebody else drops it, in which case they better already be dead). It uses light ammo and "fires almost as quickly" as an SMG, with much less frequent reloading. SMGs are great for tearing down structures in a hurry, and it sounds like the minigun will be even better suited for those purposes. Also for tearing down unwitting players who sincerely believed they were safe behind their brick walls.
Patch 2.4