Have any of you ever owned a BB gun? A BB gun is just a shotgun shooting one pellet very slowly. Amazingly it is accurate! Speed that shot up and you have a smooth bore Brown Bess musket, which is amazingly inaccurate. That is because SPEED makes the round shot diverge.
You can ponder group impact, shoving from the back of the shot column, elastic recoil, and air drag until you are old and grey. They still will not validate a theory for consistently opening a pattern from a shotgun. Those factors are minimal.
If drag or jostling were the primary factors in pattern formation, what greater opportunity could be presented than by shooting a very slow shot load. I had a friend who did exactly that many years ago. Back when you could provide your own shells at a turkey shoot, he showed up with a bag of shells so slow that he had to aim over the target to hit it. You could SEE the shot column in the air, and it literally ate up the targets. His shot column did not disperse because of drag. It stayed together, and almost every shot hit the target. After he won about six straight rounds, the local club barred his shells and thereafter made everyone shoot the same shells, which they provided.
To open patterns you need speed, speed, speed.