I seem to recall some tests the NRA did a good while back. They took, IIRC, a Marlin Goose gun with a 36" barrel and chrono'd it, then picked up the hacksaw. Cutting the barrel in 1 inch increments, they'd shoot a field load,chronograph 10 shots, then cut again.
Between 18 and 30", there was more variation between shots at a given length than between 18 and 30". Velocities at 16" almost mirrored those at 36".