This is described in the game as:
A Vietnam war vet, still seen in action today
You can find this near the middle of Remanzo province, at the edge of a small town. A nearby mission “The Mule Doctors” may get in the way.
There’s some cartel around the edge of the town, so if you can take them out quietly you can get the gun easily. The weapons case is upstairs in a building and you have to go to the left of the entrance from the road, and follow the building all the way around to find the steps up on the outside. You’ll probably be seen here if you haven’t killed nearby enemies first.
This is an automatic handgun, not an SMG. It has a high rate of fire and is a bit of a handful – too much for me! It has no accessory options at all so can’t be fitted with a suppressor, but it comes with a folding buttstock.
It has a 30 round standard magazine, no extended mag option. (Personally I prefer a pistol)