I have unlocked the Grenade Launcher attachment a while ago, and my m16a4 constantly has the underslung rail attached to it, but I can't seem to figure out how to actually use the grenade launcher.
I have looked in the control menu in the options, and it lets me set a key for using underslung weapons, but when I use it it simply doesn't do anything. (also the default hotkey for it is "3", which overlaps with the medkit.
How do I use underslung weapons?