Details
Some misconceptions are presented here.
It is true that a choke will influence the pattern of barrels of any length, and generally a tighter choke will make a tighter pattern to the limit that the shell contents can withstand i.e. lead hardness, pellet size, wad construction and resilience etc.
A short barrel can be made to pattern very tightly for turkey hunting, since I have made 3" 12 ga. barrels (20-22" typical) to pattern near 150 Winchester lead #6 turkey loads and near 200 of hevi-13 #6 (same choke) in a 10" circle when shooting from behind the 40 yard line. I believe that is fairly good for that length and shell type, but expect that a longer barrel would be able to make tighter center density at the expense of handling qualities for turkey hunting. With performance like that, the hunter can have range and handling in the same package.
The very tightest performance barrels (on average) for the turkey shoot matches will normally have the best patterns fired from longer barrels, not typically from the shorter barrels- if workmanship and choke quality are equivalent.
One local match gun builder (retired precision machinist) that coincidentally has me assist him to make difficult mechanical repairs said that he had tried barrels at 40", 38", and 36" including by re-installing the choke as he cut off 2" per time, and said that little difference was noted at 40" to 38", but the 36" was a noticeable difference with the same match choke.
Anybody that says barrel length has no input to influence the pattern when choking (especially at the ultimate end of tightness) is in error. At more moderate choke levels, the pattern difference vs. barrel length will be expected to have similar pattern density but may require a bit more constriction in the shorter barrels to negate the gas pressure influence that would otherwise slightly open the pattern in the shorter barrel.
This is the conclusion that will play out in average experiments that may not be noticed when testing one barrel against another due to the variances in aforesaid workmanship and other factors that may be too subtle to visually notice or easily measure (bore finish influences, choke alignment and surface condition, etc.).
In other words, comparing a 22" BPS against a 28" Mossberg might have the shorter barrel outshooting the longer barrel, but then you might have another opposite conclusion with a different set of test subjects. One individual against another is not going to demonstrate averages, so no anecdotal conclusions are worthy of comment.
It is easy to understand that as barrel length increases, the contained powder gas has expanded more and consequently dropped in pressure as the distance from ignition increases, so less pressure is jetting out of the muzzle when the gas seal clears the crown edge. That gas jet does accelerate when freed of containment and blows rather hard against the wad and shot column- thus disrupting the pattern and increasing the initial spread.
Longer barrels with less contained pressure makes less disruption due to released gas jet effect and that is an inescapable fact. The level of influence change may be subtle, but greater length change must have greater pressure change- when all else is equal.
It can be said that moderate length differences are not likely to have any easily discerned change in patterns without getting any argument from me.
I believe it was Churchill XXV 25" barreled doubles that were touted for handling long ago, but was admitted that they required more "points" (thousandths of an inch) to make the pattern percentages match the prevailing 30" barrels most ordered in that era.
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