The rifling will make the pellets rotate and over distance develop a big gap/void in the middle, like a donut.
I have a 12 gauge with rifled barrel and at @20yds there is a hole in the center of the pattern that will leave a duck unharmed!
Oh and did i say that a normal 20yds pattern from a smoothbore will cover the lid of a 55gal drum, the rifle pattern will cover a man sized target at the same distance.