when your shot goes out the bbl, it spreads. 50% goes above the bead and 50% goes below. hunters like 50-50 , because if a bird dips in flight, he'll still get him.
skeet shooters like to see the bird the whole time, so they prefer to have their guns pattern 60 above and 40 below for better hits. trap shooters like 70-30 since trap birds rise and by the time the shot arrives at the bird as its going away, the bird rises even more, so having a 70-30, the shot goes high ahead of the bird and the bird flies into the bottom of the swarm. make sense?
