1) Some underbarrel weapons can accept an underbarrel weapon slot, resulting in infinite weapons within a single weapon.
2) one can add underbarrel weapons to unarmed attacks and other weapons that shouldn't have an 'under' slot - like Krime gloves, katanas, etc.
3) adding an underbarrel grenade launcher to a weapon, as expected, consumes that weapons under slot. Since the underbarrel GL has its own under slot, one can add a foregrip, bipod, tripod, etc. to the UGL. However, the RC doesn't carry over to the main weapon.
binnsr
commented
2 years ago 
Three in one, actually..
1) Some underbarrel weapons can accept an underbarrel weapon slot, resulting in infinite weapons within a single weapon. 2) one can add underbarrel weapons to unarmed attacks and other weapons that shouldn't have an 'under' slot - like Krime gloves, katanas, etc. 3) adding an underbarrel grenade launcher to a weapon, as expected, consumes that weapons under slot. Since the underbarrel GL has its own under slot, one can add a foregrip, bipod, tripod, etc. to the UGL. However, the RC doesn't carry over to the main weapon.