It's a bead sight but I installed a weaver rail and a 30mm red dot scope. Getting older and it's getting harder to see little objects like that bead sight.You can buy a rifled choke tube and shoot SABOT's for more accuracy. Is it a rifle sighted barrel or bead sight? I dont think #4 buck is going to drop a deer. In Ohio, we are limited to slugs only anyways, not sure abotu your state. With slugs in an open sighted gun, I would keep shots under 100 yards and with a bead sight, down to 50.
I would be willing to bet that the problem was excessive leading rather than a slug cutting rifling into the barrel. Slugs are made of VERY soft lead and they are normaly undersized. The ignition of the round makes the skirt of the slug seal against the bore of the barrel.if you use rifles slugs, after a while of shooting it, it will wear a sorta rifling in the barrel and then using shot out of it will be no good... my grandpa had a shotgun he shot alot of slugs out of it... we tried to shoot a pattern out of it... and there was a huge blank spot in the center.. where in reality, it should have been covered with shot... the rifling was twisting the shot and throwing it everywhere... if you are only going to use the shotgun for slugs i would say go ahead.. if not.. i would say a new barrel.. my .02