Steel waterfowl loads use a slower burning powder than most turkey or upland loads.
You will get a full burn for turkey or upland loads in about 18" of barrel but normally 26" is required for steel waterfowl loads for a full burn.
All loads are also affected by the ambient temperature as well as the temperature of the shells, barrel, etc. Since manufacturers test muzzle velocity using 30" barrels at 3' from the muzzle and normally not in cold temperatures, your real world velocities are reduced by colder temperatures and of course by barrel length.
The retained energy of the shot, it's killing power, can be reduced substantially when using a shorter barrel if the powder does not get a full burn. When you add to that the effect of colder temperatures, the effectiveness of the shell can be reduced.
It's for those reasons and some others that waterfowl loads normally perform better in longer barrelled guns.
Lastly, since we normally hunt waterfowl in blinds or pits with other hunters present, a longer barrelled gun will be greatly appreciated by our fellow hunters since the blast is not quite as bad.
Thanks,
Clark
