You can never have too much gun, especially for squirrels. I like the never released 20mm Eargensplitten Loudenboomber Mark II at over 20,000 fps
On a serious note (kinda) I've been shooting 170gr Sierras over either 3031 or H335 for many years. I do my job, the rifle does its. I've used 150gr Sierra and Speer and didn't have a problem either. LE bullets don't like my rifle - factory or had loads.