A lot depends on the gun's bore, and the ammo you use. I once had a 20" barreled Ithaca Deerslayer smoothbore which would hold one lot of Remington Foster-type slugs in a 3" group at 100 yards. BUT that gun never shot any other ammo nearly as well. In those days, the Deerslayer bore was reamed to about .69" in diameter because the slugs then (except for Brennekes) were made way undersize compared to a standard 12-ga. bore. So Ithaca made undersize bores to fit the undersize slugs, and the result was good accuracy.

I recommend a 2.5X fixed-power scope with long eye relief so you can mount it forward enough that it won't take your eye out due to recoil. Then buy some slugs of various makes, and test fire them in your gun at 50 yards! Buy more of whatever slug shoots best in your gun, and give away the others! If you can get a 3" group @ 50 yards, and zero your gun to hit three inches above the point of aim at that distance, you should be able to hit a deer as far out as 100 yards, with four out of fiveshots, with a dead-on hold. But I think 100 yards is about maximum in terms of hitting it in the right spot, and in terms of having enough ooompf left to kill the thing. (Those big, fat slugs lose velocityand energy very close to the muzzle....) DON'T LET that recoil start you to flinching!!