Speaking from a long experience working with shotgun barrels, certain circumstances prove that barrel length has an effect on pattern density and dispersal.
Turkey hunting and turkey match shooting where maximum density patterns are desired will, when all else is equivalent, tend to group more tightly with a longer barrel such as 30" vs. 20".
The 20" barrel is much more maneuverable for hunting and is a typically better choice for turkey hunting as an example.
Shorter barrels may need a slightly tighter amount of choke to get equivalent results of a longer barrel due to the shot spread effect from the muzzle pressure that naturally drops as length increases.
It is true that most times the pattern, velocity, and hitting power will be little changed by modest barrel length differences, but can be radically different due to quality of bore contours, finish, choke alignment, and the details that make a fine barrel different from an economy barrel. However, shotguns being a fickle servant, we may sometimes find that what was considered likely a plow horse wins the triple crown in pattern density at long range. Thus is the nature of the beast- that no small sample comparison will ever show much to be able to conclude with certainty about most analyzed details. Some details will require much more exposure than anecdotal levels to reduce an uncertainty factor.
[email protected]