The air rifle idea was my original thought but I'm unsure how much punch they have out at 25-30 yards away. When i was a kid (a long while ago), we'd shoot those ground squirrels with fairly potent stuff and they are durable little critters. My favorite was a .30-06 "Accelerator" (.223 sabot) but that's a whole different kind of squirrel misting...

One that came to mind was the Chiappa "little badger". As far as I can tell from my research on Calguns, threaded muzzles are OK in CA on rifles (but are no-go on pistols). I've shot one of the Chiappa's once and it seemed like it may fit the bill.

Bear in mind: once the last squirrel is dispatched, the rifle will never be fired again, so there's a limit to how uppity this needs to be.