If you're looking for a breech-loaded grenade launcher in Destiny 2, it's pretty likely that you're trying to complete the newly added Climbing the Mountaintop quest to get The Mountaintop grenade launcher. In the quest you need to rapidly defeat a whole lot of enemies with breech-loaded grenade launchers—bonus progress in Onslaught—or bring one into PvP to kill guardians.
For those who are new to Destiny 2, or just haven't ever really paid attention to weapon types, you might be scratching your head pondering when such a complicated sounding gun was added to the game. Don't worry; unless you have literally just started, chances are you already have a breech-loaded grenade launcher, and even if you don't, it's easy enough to get a decent one.
