I just wrote this the other day...
With rifled slugs in a smoothbore I like a red dot, but with the longer range, and better accuracy of sabot slugs from a rifled barrel I like a 2x-4x variable.
THE most important thing to look for in a sluggun scope, aside from durability, is eye relief. You want to be able to see through the tube without being right up near it. Bushnell makes a Banner line 2-4x with 6" of eye relief.
Keep the scope on low power for fast shots up close, and turn it up if you have time for a longer one.
Red dots have HUGE eye relief since they have no magnification, and can be used anytime you can see through the tube. I'd take advantage of th epotential accuracy of the good barrel purchase you made and go with some kind of magnifying scope.