Although Hastings gives a written warning about using Remington Copper Solids in their barrels, I've found that that particular round groups best at 100 yards with their barrel. Go figure. BTW, the one with the cantilever scope mount is best by far.
All other slugs I've tried just don't seem to like the rifling. Some even tumble through the targets.
Just remember, when you've sighted in at 100 yards and are feeling comfortable with your shooting, you'll need to hold your crosshairs quite a bit under your intended entry point at a target standing at 20 yards. Slugs drop a considerable distance at 100 yards, requiring you to compensate at short distances. Stay away from shots over 100 yards unless you've practiced them.
Put a sling on that shotgun and use it for all of your shots, wrapping it around your brace hand forearm. When not shooting offhand in the field, put your barrel against a solid surface when sighting (like a tree) and exhale slowly as you squeeze the trigger.
Be careful where you place your slug on a deer. Believe it or not, with a slug, a double lung shot could make for a difficult trailing job. You want the crosshairs to land high on the shoulder. When the slug hits home it will break the shoulder and the spine, dropping your target on the spot. No trailing needed.