The 5.7 USG is described in the game as:

A low-recoil, high-penetration handgun.

You’ll find this in central Ocoro province, inside Ocoro Base, a large cartel cocaine factory. The mission “Aggressive Competition” is nearby. The weapons case is on the northern edge of the base, at the back of the factory buildings, behind a shipping container.

There’s a very direct approach from the water just to the north of the base. You can come up over the wall with little to no resistance. You may need to take out some snipers on the bridge, and be wary of nearby sicario guards, but it’s more-or-less a short, straight run to the weapons case.

This is my personal favourite handgun as it achieves maximum score on the quietness scale (along with the P12, M9 and the P45T you start the game with). But unlike those handguns, this one scores better overall on pretty much everything else. It’s beaten on damage by the D-50, but that’s a very noisy cannon of a handgun.

It comes with an already great 20 round standard magazine, and a 30 round 5.7mm magazine is available and I recommend, which you can also find in Ocoro. This is a great gun, I can’t think of any drawbacks.