The grenade launcher attachment is for assault rifles, which replaces perk 1. This is available when you first unlock an assault rifle. You get 2 grenades for the launcher with the perk. With the perk Overkill, it is possible to equip two assault rifles with grenade launchers for a total for 4 grenades.
In multiplayer, it is commonly referred to las the 'Noob Tube' do to the belief by many players that it requires almost no skill.
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