Your barrel has sights on it. That tells me that you will be mounting the scope to the receiver and not the barrel, right? If that's true I don't know if you'll really gain much range by adding a scope. The best pump guns are generally those with cantilevered scope mounts. I've seen guys have lots of trouble trying to sight in pump guns with receiver mounted scopes. The barrel seems to have enough play to cause "frequent flyers".

Still may be a good idea to go to a scope, especially if your eyesight isn't the best for iron sights.

I always recommend Brenneke KO Sabots. They have worked very well for me on deer and they are cheaper than most sabots. These are actual sabot slugs. There is a KO slug that is not a sabot, so be sure to buy the sabot type if you try them.

Generally, most sabots slugs will kill deer just fine out to 100 yds. There are now quite a few that will retain enough energy to kill deer reliably out to 150-200 yds. Your effective range will be determined by your ability and the gun's accuracy.