You didn't say if you have a scope on it or not. I have a scoped 1187 12 gauge with a fully rifled barrel and I get good results with the Remington Sluggers too. You should be able to dial it in for 2 inch groups at at 75 yards but in my opinion anything above and out to 120 yards is stretching it. I sight mine in 2 inches high at 50 yards. My slugger drops right at 4 inches at 100 yards so, 2 inches low at 100 yards. I have shot further than 100 yards but just once. It was roughly 137 on a big doe, I held high on the back and hit right in the heart/lungs. I'm not sure on the flight of a 20 gauge.