I don't know about California ground squirrels but the Washington variety find a .177 consistently fatal. I have only used a .177 on them. I appreciate the flat trajectory out to forty yards or better - I needn't compensate for drop. Even at forty yards it will ventilate a ground squirrel's head.If I were hunting tree squirrels the .22 would definitely be the caliber of choice. I think they're hardier than their grounded cousins.