Miller I think my reasoning is perfectly sound on this one.

If you slow the rate of acceleration of the wad, you also slow the rate of acceleration of the shot charge. The wad is pushing it to ever higher velocity and without a push it will go no faster (Newton).

Note that we are not decelerating. Positive pressure is present until the barrel is vented to atmosphere at the muzzle.

It's well known that barrel length does affect velocity to the tune of about 15-20 fps per inch, so that proves continuing acceleration.

The 'retarding' or 'stripping off' concept is bunk, cherished lore though it may be.

"The price of good shotgunnery is constant practice" - Fred Kimble