Need more information. Did H&R shoot the gun with the scope? Did they inform you which Federal Sabot they used? Are you shooting off of a solid bench with sandbags front and rear?
Rifled slug guns do not preform like rifles and if your gun truely shoots a 1" group at 100 yards then you have an exception gun. I have dealt with 12-15 rifled slugs guns over the last decade, pumps, autos, bolts and single shots from Benellis, Brownings, Remingtons to Mossbergs and Savages. Most will shoot in the 3-4" area at 100 yards. I have one that I kept that will shoot about 1 1/2" @ 100 on a good day when I'm doing everything right.
I would find out what ammo they used. Make sure everything is perfect with the scope and bases(I have sent two Leupold scopes back to be fixed). Deep clean the barrel by 1st using J&B paste and a Tornado brush, then Hoppes/shooters Choice/Butches Bore shine and repeat until patches comes out perfectly clean, then an extremely light coat of Kroil oil. Store barrel down in your cabinet so any excess oil will run down the barrel and not into your action or stock. Head to the range on a nice cool morning with little or no wind and low humidity. Sandbag that gun front and rear and cushion your shoulder. Start out at 25 yards, move to 50 and sight in dead on at 75 unless your strickly shooting a field from a stand then sight in at 100. Take a bore snake, solvent, patches and cleaning rod with you. Clean the barrel every 3-5 shots and let the barrel cool an honest 10-15 minutes between shots. Heat and fouling build up extremely quickly in slug guns and ruin accuracy. You are asking a lot of a small piece of plastic(sabot) traveling at 1400-2000fps. Good luck.