You need to invest in a good choke. I just bought a new 870 super mag and shoot Estate 3" 1 3/8 #3 and killin em dead at 40&50 yards. I got a mid range and long range(when it's windy). With the log range over the decoys, you have to shoot like you would a rifle(LOL). They're worth the money. Shells aren't cheap.
