B-Square has mounts for your Mossberg 500 12 Ga. They do not come with rings. If you want to scope the shotgun they have rings for the mount also. You could also put a good quality dot unit on it like and EOTech Holographic, Pro Point which are a little expensive but you get what you pay for! And there are some other decent red dots for less. But be cautious when buying one of those. There is a lot of junk out there also! Some have so much parallax in them they are next to useless!
You may want to get good advise from one of he other members here on the FTF. 03