Lot's of choices
Back in the day before I got good shooting them from afar with rifles I used to use 12 and 20 gauge shotguns with anything I had from 6 to 8 shot with 6 being the better choice. A 12 is overly big and heavy for most long walks. A 20 gauge with full choke will cleanly kill squirrels out to 30 to 40 yards with 1 oz of lead #6 from a good full choke. Now shooting steel only changes the equation. You can probably find #4, 3 inch steel for 20 gauge and it should do the trick. 2 and 3/4 inch #4 steel should work for 12 gauge. With steel use modified and not full choke. Don't pepper your well by shooting so far away your pattern opens up. Really a scoped 17 caliber rifle would be a better choice and you can get lead free ammo. You'll be able to take them from far enough away they won't know what hit them. rc
Back in the day before I got good shooting them from afar with rifles I used to use 12 and 20 gauge shotguns with anything I had from 6 to 8 shot with 6 being the better choice. A 12 is overly big and heavy for most long walks. A 20 gauge with full choke will cleanly kill squirrels out to 30 to 40 yards with 1 oz of lead #6 from a good full choke. Now shooting steel only changes the equation. You can probably find #4, 3 inch steel for 20 gauge and it should do the trick. 2 and 3/4 inch #4 steel should work for 12 gauge. With steel use modified and not full choke. Don't pepper your well by shooting so far away your pattern opens up. Really a scoped 17 caliber rifle would be a better choice and you can get lead free ammo. You'll be able to take them from far enough away they won't know what hit them. rc