Welcome to our campfire, Pard!
"CAS" stands for "Cowboy Action Shooting[tm]" which is a registered trademark of the Single Action Shooting Society, although it gets used generically without the trademark attribution I put in the brackets, above. As long as you aren't trying to make money on it, you don't have to worry too much about using the term or the abbreviation. (In my book, "Trailrider's Guide to Cowboy Action Shooting[tm]", I had to use the trademark, with permission. (I'm not trying to sell the book to you. A LOT of changes have been made since I wrote it. There are still some good points about equipment, costumes, etc., but the rules have changed, as have the classifications. Wouldn't want to confuse you too much.
Savage 311 in any gauge larger than .410 is fine so long as the shotgun doesn't have automatic ejectors (extractors are okay...the difference being ejectors throw the shells clear of the gun, whereas extractors just move the shells back far enough for your fingernails to get ahold of the rims).
SASS Headquarters will get back to you eventually. They had a real shock recently, when Coyote Calhoun died suddenly in the office, and I imagine they are picking up the pieces.
Have fun at the shoot!