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.
What is a breech-loaded grenade launcher?
There are two types of grenade launchers in Destiny 2: power grenade launchers and breech-loaded grenade launchers. While the first type goes in your heavy slot and uses heavy ammo, breech-loaded grenade launchers go in one of your first two weapon slots and use special ammo instead.
If you're a new player who doesn't have one yet, the easiest to grab is Empty Vessel through Zavala's Focused Decoding. Of course, if you're trying to get those rapid final blows for The Mountaintop quest, you'll want something with more of an AoE or a wave frame breech-loaded grenade launcher so it's easier to get lots of kills at once.
You can grab Witherhoard from the Monument to Lost Lights in The Tower in the Shadowkeep section, provided you have an Exotic Cipher and an Ascendant Shard—this is what I used to complete the quest and it was real speedy. If you happen to already have Fighting Lion, this is another good option for completing this quest since it's the only primary breech-loaded grenade launcher, and so has unlimited ammo.
If you're lacking the materials to get Witherhoard, your best option is to complete the Vox Obscura exotic mission, which ufa747 is active in the Legends tab this week. This will get you a craftable Dead Messenger; a very good exotic breech-loaded grenade launcher that you can try out in PvP, or bring into Onslaught to complete the quest either way.