Still have my 500 Moss 12 ga. and I use:

3" Whinchester Heavy Load #5's in the Undertaker H.S. that others on this site recomended to me. Bass Pro has it for $16 but found it later at Wal Mart for $14. Did a feild test on a tree at around 40 yards on a plastic bottle and I loved the pattern I got not only on the bottle but the tree as well.