In the field of gas dynamic, this would be refered to as a convergent/divergent nozzle - commonly used as a rocket motor. The convergtent section is designed to accelerate the gas to its sonic veocity followed by additional acceleration in the divergent section; note that the convergent section can't accelerate the fluid to above sonic velocity. Who knows what the sonic velocity of a shot swarm is when it is acting as a semi-fluid - - - not me!!!
I'd expect these chokes to act enough like any choke of equal constriction as to be indestinguishable. Winchester WS-1 skeet #1 is of the convergent/divergent design. I've not been able to find any benefit or liability in patterns from this design.
DDA
