The SR3M is described as:
A compact, full-auto Russian rifle.
You’ll find this in western Agua Verde in Alicia Town. The area is full of sicarios. The weapons case is outside a house, in the yard next to a brick wall.
I followed the GPS through the town to the street by the houses with the target building. There are a lot of sicarios here patrolling around and wandering the streets, in and out of buildings. I couldn’t find a quiet, stealthy approach for this, so we ended up going loud and just gunning all the narcos down in the street. You can then run around the wall of the yard to walk in the entrance, or climb over via a dumpster near the weapons case.
The SR3M is rated as the worst assault rifle for noise reduction, penetration and range. Possibly because it has a high rate of fire and scores joint top for handling, alongside the AUG A3 (found in Barvechos), making it really more like an SMG than an AR.
Because this gun has its own foregrip, you don’t need to unlock any to equip it as far as I know. But you also can’t remove it to fit a different or better foregrip under the barrel either, unfortunately.
This gun is chambered for a 9×39 round and has a 20 round standard magazine which is usual for assault rifles, with the option to increase this by 50% to a 30 round extended magazine (in Agua Verde), but there’s no large magazine available. For me, I like an AR to be an AR and an SMG to be an SMG, so I’m afraid I don’t see a use case for this weapon in my load out.