[RPKESQ]

Aerodynamics. That is the key.

Pellets as used in a shotshell do not behave as a single streamlined (or even partially streamlined) spinning projectile. In a shotgun all dispersion is caused by a single factor. Shot deformation.

Speaking of only modern smokeless powder and modern shot cups we obtain difference shot dispersion (pattern size) by choke (more or less of it). Barrel length has nothing to do with it, or so very little as to be completely insignificant. Especially with modern shot cups which protect the pellets from deformation so well that barrel length is of no concern.

A properly constructed choke will affect shot dispersion. It does so by altering the "stack" length of the pellets in the bore. The greater length this mass of pellets has the tighter the pattern will be. When pellets are stacked up behind each other the air has the most effect on the leading pellets and the trailing pellets are protected to various degrees from the effect of aerodynamic drag. This means the leading pellets "break trail" for the following pellets in a manner of speaking and allow them to fly truer, hence less dispersion.

In a more open choke the pellets are not stacked behind each other as much and therefore the air drag on each one of them is greater.

Since not two pellets are exactly alike, the aerodynamic drag and vectoring due to surface irregularities on each pellet result in an independent flight path when exposed to the air. This is the cause of all shot dispersion. The choke actually only affects the "presentation" of the pellets to the air. The air drag is the actual dispersion causality.

Can you make a full choke gun spread to a wide pattern without altering the choke? Of course, just mangle the pellets more in the load. Can you make a cylinder choke shoot tight patterns? Again, of course, just make the pellets less deformed. It really is pretty simple.

Ask yourself this. In a pistol or rifle on a test bench, does barrel length affect accuracy? Of course not (until you look at increased velocity from slower burning powders at long ranch, aerodynamics again).

So back to my original example. If you built test barrels with exactly the same internal dimensions and used the same type of shot cup, pellet composition and powder, case, etc. over a sample of 10 shots out to 40 yards you would not be able to have a statistical significant variation between various barrel lengths. Since the distance between even a 1" barrel and a 36" barrel is not very much % wise to the shot distance (40 meters), any barrel length effect would be hidden in the random shot dispersion caused by each pellet's unique aerodynamic profile.

If you want to have more dispersion from your tactical shotgun, load it with high drag separators and no shotcup with pre-distorted pellets. But you pay the price once you get beyond 15 meters with a significantly lower hit probability with standard 00 or 000 buckshot pellets loads (pellets per load).

Competitors can often “game” the system by using different chokes at different stations, long or extended chokes are not necessary if the original barrel has a proper bore and choke construction. Modern shotcups have changed the patterning of all sizes of shot over the last 35-40 years. So many shotguns can benefit from bore increases and choke as well as forcing cone alterations. This is really just matching the bore to the more modern loads. A shotgun still works the same as it always did. Many European shotguns still have the short forcing cones and tight bores and short chokes required for the older shotshell construction. This older construction is still in use and in some case favored over the modern high-strength plastic shotcup for two reasons mainly. Paper or cardboard wads are cheaper and more environmentally sound than the plastic shotcups.
Gunsmiths have had a lucrative business altering older and European shotguns to the “new” bore, forcing cone and choke profiles. I say “new” because all of this was known 75- 100 years ago. There has been very little really new in shotguns for a very long time. Try looking at the British Patent Office for firearm patents the period of 1860 to 1930. You will be amazed at what they knew and could do.