The MPX is described as:
A quality SMG with decent range.
You can pick it up north-west of central Mojocoyo province, at a Santa Blanca cocaine hideout. The “Western Cocaine Cache” story mission is nearby. The weapons case is on the balcony of a building on the north side of the hideout.
If you approach from the road on the north-east, then after you take out the snipers in the towers, and any straggly enemies near the entrance to the base, you can sneak in the entrance and round the inside of the perimeter to the right to get to the building. If you’re careful not to be seen, you should be able to get in the building up the stairs and to the weapons case.
The stats for this gun are very similar to the MP5, but with a lower rate of fire. It’s a fairly average SMG by all accounts. The recoil pattern is manageable, kicking mainly upwards and to the right a little.
It gets the usual 20 round magazine which is ok with the slower rate of fire. A 30 round extended mag is also available and can be found in Media Luna which always helps.