You've described momentum well, 2-piper.

Understand though that I'm talking about a reduction in acceleration, not a reduction in velocity.

If you were to vent the pressure off prior to bore exit then not only would acceleration stop but deceleration would begin due to bore friction and air drag.

A powderless shotshell does exactly that. The wad sticks for the reason you've described.

A properly functioning shell though accelerates the payload all the way to bore exit and internal ballistic graphs show this well.

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