Okay so yeah, this adds two new types of 12Ga rounds the game (I mean really, who would put that in a 20Ga?) with the intent of making
the shotguns more end game worthy as well other for other reasons like wanting more pop, also includes a unique Hunting Shotgun.
The ammo and new gun have been added to their respective lists so you can buy the ammo from some merchants and repair the shotgun with others.
These rounds are intended for tough enemies (especially armoured ones) so I don't suggest wasting them on weaklings.
A more in depth ReadMe file is inside the rar so be sure to read it, install as is simple as it's just the esm and uninstall is just as
simple.Uses vanilla game content so no DLC are needed, shouldn't conflict with anything else that I know of, optional file version requires Gun Runners DLC!
Edit: Sorry it was kind of late when I uploaded this, wrote the location of the weapon in the ReadMe but forgot
to mention it here, the unique shotgun and some ammo can be found in the Old Nuclear Test Site Shack.

UPDATE: As per the suggestion of user JLF65 I have added Depleted Uranium as a misc item that can be found in the game world
to be used at work/ammo benches for making the DU shells, they can be found in each vault near their respective reactor or systems room.
These containers (as seen above in images) are enclave crates with a RAD sign on them and are named Lead Lined Crates, they
also have Rad Suits next to them so they're easy to spot, you'll need 50 science skill to breakdown the DU and 50 gun skill to craft the ammo.
There are 12 of these crates spread through the Mojave, some are in vaults while others are in other appropriate areas.