To the OP, If you have a trap range anywhere near you, you can normally go there and buy once fired hulls. I would suggest Remington Premier either the green ones or gold.
Then a light 1oz load using Hodgdon Clays powder 17.0 grains Winchester 209 primers and Remington TGT-12 wads. This is a load that my wife & I shoot all the time either shooting sporting clays or at rooster. For sporting clays we use #8 shot & for roosters we shoot #5.
