A shotgun is probably a better choice with all the leaves. I think a 12 gauge is overkill. I prefer a .22 but when I shoot a shotgun, it is a 20 gauge. If you are shooting lead shot, #6 will do the job. I like #4 for non-toxic loads.
PS...I know there are squirrel dog people in OK. See if these guys are close to you:
Sam Allar
Whiskey Creek Feist
http://www.whiskeycreekfeist.com/
Good luck and welcome to the "squirrel huntin' addiction" !
-Marc