Underbarrel Weapons Issues: Some underbarrel weapons cannot be attached to weapons, others do not function properly.

[EmmjayXII]

Weapons like assault rifles and longarms which should be able to have an underbarrel weapon attached cannot take either of them as an attachment. When right-clicking on the weapon and attempting to add an accessory to them, the Underbarrel Laser, Underbarrel Bola Launcher, Underbarrel Grenade Launcher, and Underbarrel Shotgun cannot be chosen.

Additionally, the Underbarrel Chainsaw and Underbarrel Flamethrower cannot accept a chainsaw or flamethrower model, respectively.

[LNSmith458]

While it isn't in the accessory list, in the meantime you can add an underbarrel grenade launcher to the weapon by right clicking on it, and selecting Add Underbarrel Weapon.