What kind of ammo kan a Jc Higgins model 20 shotgun pump action made by high standard arms shoot and can it shoot slugs?

The JC Higgins Model 20 is a well-made firearm. Provided you have it checked by a competent gunsmith, it can handle any standard 12ga 2 3/4" shotshell. If the gun is not choked 'FULL' you should be able to use slugs.