I do NOT remember those results the same way, and I have done a lot of my own velocity measurement. Velocity continues to increase up thru 32" (an maybe more), but the rate of increase drops way off. And before someone tells me the powder has burned before you get to 18", I'm not saying it hasn't, but the big bunch of compressed gasses behind the shot has not gone down in pressure to atmospheric yet, by a long shot. If they had, it wouldn't go 'bang' when it exits the barrel. That is why longer barrels are quieter though, because the pressure does drop.
I agree, the choke determines pattern.