The taper wall of the choke portion departs from the straight wall, and the shorter the the choke, the greater the angle. Yes, that is the unbalanced force (action/reaction) that shot which begins changing direction rebounding from the choke wall begins a disruption in the mass of the shot stream.
In a true cylinder bore, that shot mass is impacted by no unbalanced force, and tends to stay in closer proximity to the center line.
All of this is my conjecture, and I can be convinced I'm wrong, but what supports my belief is lots of pattern paper shows a shot density well tighter than Skeet 2, approaching modified. I never gave it enough thought but I am sure that wad type will have some effect as well. It simply makes sense, that in the absence of anything that will alter the shot direction as it travels down and out the bore, the total shot mass will continue in a straight direction.
All barrels shoot to a certain point of impact. I do have a gun with two sets of barrels; one being cut from 30" to 24", the other being more or less Imp.Cyl and Modified. I fully expect that if I draw a circle around the total pellets of the cylinder bore barrel, and then shoot at the same paper with a choked barrel, there will be new holes. Beyond some distance, this will, no doubt, be less noticeable.
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