Does your new Mossy Shotgun have screw in chokes in the Muzzle area of the Barrel or does it have a fixed Choke Barrel? You may be able to buy a Rifled Choke Tube for it.
Note that repeated use of Slugs, even the soft Foster Slugs, through a Full Choked Barrel will stress and possibly ruin a Barrel. Pressures will be at dangerous levels.
I have read that you can shoot the soft Foster Slugs through a Modified Choked Barrel, but I've never done it, so all bets are off there. Read into that, I wouldn't do it!
In general, you need to have a Cylinder or Improved Cylinder Choke in your Barrel, in order to shoot Slug through it. The soft Foster Style Slugs won't hurt most Shotgun Barrels and open Chokes.
I've used the Foster Style Slugs through smooth bore barrels with good results out to 100 yards. My old Remington 1100 has a smooth bore Slug/Riot Barrel, an open True Cylinder Choke, and has accounted for 5 Deer, so far.
That said, I would rather use a Rifle.
