COR is simply the rate at which a ball will reflect back after being cast against an immovable object. So the higher the core the higher the reflectivity or the higher the bounce if you will. Core 52 has a higher bounce than a core 40, but core is only part of the picture, although if you could choose a core 52 with low compression like 275lbs. (which would be the ultimate mush ball but very lively) this would be a good ball for a stiff walled bat like a the new ASA only Combat for example and I do have a full and detailed bat review on the New Combat Softball bats which you must see.