Anybody who read the 1994 Assault Weapons ban might have noticed that “Grenade Launcher” is one of the features listed as bad, and could have lead to the banning of your gun if it had one other “Bad” feature.
Of course reasonable people would assume it meant centerfire grenade launchers like the M203. Only the M203 is its own firearm, and an NFA item to boot.
Nope the law was talking about this relic of warfare past:
Kinda neat! Not very practical as to go from launching bullets to grenades you need to clear the rifle, chamber a blank round, and switch the gas system over to “Grenade” (which is simply a cool way of saying it blocks the gas port to the piston so the rifle won’t cycle, and all the gas will drive the grenade), THEN you must put the grenade on the launcher, arm the grenade, aim using the grenade sights, and fire.
To convert back to bullets, you need to manually clear the action as your gun should not have cycled and ejected the blank shell, switch the gas system back to its normal open position, then load the firearm with live ammo.
The newer versions (like for the past 20 years) of rifle grenades are bullet trap or bullet-through designs that launch with normal ammunition.
The things aren’t completely dead because every rifleman can be a grenadier with rifle grenades whereas with a 40mm launcher you need a gun dedicated to the role.