Back to the OP question...
I'd recommend to him to use a 20 gauge shotgun, single shot or pump, full choke, #4 or #6 shot.
Reasons are: lack of ricochet, forgiving of poor aim, reasonable expense to purchase and shoot, flexibility of ammo... slugs to bird shot...in case the OP'S needs change, can shoot at a moving target with good expectation of a hit.