I don't find too many of these on enemy troops, but the PBX Auto-Slug 4 can often be found in weapons crates. You also unlock it as a supply drop option when you secure the Riscos Altos region. Secondary fire is accomplished by pressing RB. Simply destroy a vehicle with that. It's probably easiest to down a motorcycle or basic military car; don't try for a tank. This actually unlocked for me when I blew up a chaos object in a base and the explosion took out a vehicle in collateral damage, so it doesn't seem like it's super picky about it.