The answer is that most clay and game loads nowadays are loaded with relatively fast-burning powders which release all of their energy while the shot and wad are in the first 15 to 18 inches of barrel, and after that the shot isn’t going to go any faster.
Therefore, for most of us, barrel length is of little significance in this case, and the only advantages long barrels give is that they provide the shooter with a longer sight plane and a steadier gun.
There could be some slight advantage with longer barrels if very heavy wildfowling loads are being fired – particularly with steel.
Very heavy loads require slower-burning powders.
As far as I am aware, the muzzle velocities quoted by most cartridge manufacturers are from 30-inch tubes, although exact information from all manufacturers is hard to find.