The M9 handgun is described as:
The standard issue pistol of the US Army.
You can pick it up in central Villa Verde, at a Santa Blanca makeshift runway. The weapons case is in the armoury near the helipad.
The area is split into two; as you approach on the road from the north, there’s an area with buildings, the armoury and a helipad where there are six sicarios. Then the road continues up a small incline to the runway with hangers and more enemies. You can ignore the enemies near the runway. The six enemies near the armoury are easy to take out with a couple of sync shots.
The M9 is another good all round handgun, with similar stats to the P45T you start the game with. It has a reasonable 15 round standard magazine and a 25 round extended mag is also available (also in Villa Verde).
Nothing lets this side arm down and it’s one of the quietest with only the P12 being slightly quieter (available in Tabacal), however the 5.7 USG does have better stats on the whole and is what I personally always recommend.