What are breech-loaded grenade launchers in Destiny 2?

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...
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