First off, this isnt a game breaking flaw, but it is a little puzzling. So I personally have shot a 40mm grenade launcher on a Mk.16 (FN SCAR). I know that specific gun isn't in the game, only the 7.62 Mk.17, but FN makes a grenade launcher specifically for the SCAR called the Mk. 13 eglm.
Now, the standard GL in game switches styles depending on whether you equip it on a bullpup or not, and the style fitted to the bullpup rifles is exactly like the eglm that FN makes. Why? If you're going to style the grenade launchers like real ones (like the m203), why wouldn't you allow them to be equipped on the right weapons?
I mean the style that fits on non-bullpup rifles looks almost exactly like the one the UK manufactures for the L85A2, yet the one that you can actually equip on the L85 is the one that should actually be on the SCAR! Im absolutely stumped on this! I know it's just a little thing, but I'm a huge gun nut, and this just grinds my gears.