I enjoyed FlyingSixtySix’s mod for the UPM61 (https://justcause3mods.com/mods/fully-automatic-upm61/), but it didn’t work with the unlimited ammo mod I use because they share a file. So I edited it. All credit to FlyingSixtySix, literally all I’ve done is changed a 0 to a 1 and uploaded it for convenience.
Halves the fire rate and turns on automatic rate of fire. Works out of the box!