It's not a venturi, a water hose, or cars at a stoplight.
It's a shotgun choke. Analogies are not exact.
If the pictures on page 3 of this thread do not convince you that the shot charge fired from a choked barrel lengthens between 6 feet and 14 feet then nothing will. The only possible thing that could cause this is speed differential. The trailing pellets are going slower than the leading ones in spite of the drag front acting on the leading pellets. The trailing pellets must have been launched at lower velocity.
If there were an endless constant stream of pellets traveling through the choke, they would all behave the same at exit. This is clearly not the case in the physical shotgun.
The 18 foot photo shows the pellets starting to bunch up and spread due to the same effects that work on the cylinder choke shot mass. That's 6 yards downrange and still only 1/3 the diameter of the cylinder choke sample.
That is 'choke effect' plain and simple and that's all there is to it. It's caused by a differential in speed between the leading and trailing pellets. If all the pellets shown in the 6 foot picture of the choked barrel were going the same speed, the pattern would already have started to mushroom like the photo above it.
Here's a WS-1 choke, if this site will allow me to post the link to another shotgun site...
http://www.shotgunworld.com/bbs/viewtopic.php?f=60&t=365315
