Stan, I doubt that the physical/actual constriction of the shot column actually does much it the way of choke effect. The effect, IMO, comes from lower pressure within the shot column. This lowers "sideways wind" (good bit of air within the shot column) that gives the pellets an initial sideways velocity (low velocity, but sideways none the less) and lowers the "spring apart" of the pellets (due to compression).

DDA