Drew, the experimental data is simply being misinterpreted on the short chamber comment. He's putting forth one theory, which cannot be correct because of the constant acceleration in the bore.

We're talking hundreds of G's here. No pellets are going to fly ahead of the end of the shot stack while in the first part of the bore. The shot column does transition from cartridge diameter to bore diameter, but it enters the choke section as a cylinder.

The most likely answer to his observed increase in pattern density, and indeed the only possible one, is that the shell mouth interference distorted the spherical shape of the rather large (#4) pellets less than the control load in a standard length chamber.

The odd shape pellets that pattern so tightly demonstrate the predominance of aerodynamic effects on shot dispersal. Those pellets quickly stabilize in flight with the projections acting as fletching on an arrow. They don't tumble constantly like the scrubbed lead pellets do that have flat sides.

Brent, as I've tried to explain it's aerodynamic forces that spread a shot cloud. The speed differential I refer to is pellet to pellet speed difference. All the shot is not accelerated to the same velocity by the choke. The leading pellets are going faster, and retain that differential for a good distance as evidenced by the continuing lengthening of the shot stream as shown in the photos. Even with the increased drag on the leading pellets and the following ones flying in their 'draft' it takes quite a bit of travel for the dispersal to equal what a cylinder choke shot cloud looks like at a shorter range.

Profiles of common skeet chokes are available on the interweb.

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