The AFG is described as:
A foregrip that assists in muzzle control, but adds some weight.
You can find it in north-east Barvechos province at a cartel road checkpoint. The accessory case is in one of the checkpoint buildings, on a workbench.
There are four sicarios and a sniper in a tower here, but the enemies are evenly spaced and simple to eliminate.
There are four foregrips available which improve handling and reduce accuracy to varying degrees. The AFG does the opposite, and improves accuracy while reducing handling. If – like me – that’s your goal, then this is probably what you want on your underbarrel for certain guns.
The upgrade to this is the Shift Short Angled Grip (also in Barvechos), so you might want to check that out. Alternatively, for assault rifles the only other underbarrel option is a grenade launcher. If that’s your bag, take a look at the M203.