I will use just about anything, but my preference is standard velocity (not so loud, and it does not scare them as bad if you miss, and you might get a second shot) solids. The hollow points can mess up some meat. My current squirrel ammo is an old (20 year maybe) batch of Remington Target. I also use a tool that I bought to flatten the nose so it looks and acts like a wadcutter. It does not cause it to expand, but it does look like you hit the squirrel with a "hole punch" because it does not allow the tissue to expand around the bullet as it passes through. Works well enough for me.