Describe the solution you'd like:
Add the ability to modify some semi-automatic and automatic rifles in the gun modification table to attach them a grenade launcher module.

The attached grenade launcher will allow the player to launch previously loaded grenades (only allows the loading of a single grenade at a time) or the use of large cartridges, by pressing a specific configurable button (I suggest the middle mouse button for default), as an additional function of the modified rifle.
But the attached grenade launcher will also make the weapon heavier, making it slightly slower to move.

This modification would be expensive.

Describe alternatives you've considered:
Include an special rifle with attached grenade launcher in some modern-weapon like content pack, or inside an specific mini-content pack.

Additional context:
Example of real attached grenade launcher:
https://en.wikipedia.org/wiki/M203_grenade_launcher