You can and should shoot buckshot through the rifle sighted barrel. It should do well with it. If it is a new gun the barrel will be choked improved cylinder from the factory. If you bought it used, it might be one of the older cylinder bore 20" barrels. Either way, as long as it is a smoothbore, you are good to go with either buckshot or slugs. Shotgun barrels with rifled bores tend to do well with sabot type slugs only, they often throw buckshot patterns with holes in the middle.
Remington marks their new short barrels with IMP CYL stamped into the left side of the chamber end, looking at it with the muzzle pointing away from you. Give yours a look and see if it is marked. I have had good luck with these barrels, they usually deliver better patterns than the older cylinder bore barrels and produce useable patterns at somewhat longer range.
Try patterning with 00 buck first, the new low recoil/reduced recoil/managed recoil loads available from several manufacturers these days are a lot easier on the shoulder. Then see if there is a size the barrel likes better, I prefer not to go smaller than #1 buck and really prefer that if a gun will pattern it well- you get higher pellet counts in #1 buck loads without sacrificing pellet size too much. See if there is a particular load your barrel seems to like better, if you want tight patterns that is. Some folks want their shotguns to be true scatterguns- if that's what you want, try S&B 00 buck in the clear hull and you should get exactly what you are looking for. Any standard velocity buckshot load with soft lead pellets in an unbuffered load is likely to give lots more open patterns than hard lead/plated shot in a buffered load launched at reduced velocities.
I sight in with slugs when using a barrel with sights, as a general rule. My preferred slug is the Brenneke KO loaded by Kent, it is a hard lead sharp-shouldered slug that cuts full diameter holes and is usually quite accurate as well. They are often found on sale at reasonable prices, too. Buckshot loads are more forgiving at longer ranges but still will impact in a single cluster close up. After you sight in with slugs, try your favorite buckshot load and see where it prints in relation to your slug zero at various ranges. Often the two are usefully close but it is not a given- check it to be sure, you might want to change brands/loads to get closer points of impact with slugs and buck.
Stay safe,
lpl/nc
