I suggest having weapons that support rifle grenades display the grenade launcher adapter on the muzzle at all times. Currently, it is only visible when the grenade firemode is selected, which I think is a huge shame, as it changes the look and "vibe" of the gun quite nicely.

Why add it?

Adding more variety and character to weapons
"Justifies" having separate GL versions of guns in the game and differentiate them visually at a glance
Realism - in real life, you would keep the adapter mounted at all times, not mount it only to fire off a grenade
Grenade adapter is already modelled in-game

M1 Carbine with attached grenade adapter (and no mounted grenade). Currently, you can only see the adapter properly if the grenade firemode is selected, and no grenade is loaded. My suggestion is to have the adapter visible 100% of the time.