Two great resources on this topic:
Sporting Shotgun Performance, Dr. A.C. Jones
The Mysteries of Shotgun Patterns, George Oberfell & Charles Thompson
Both books do a good job put some quantitative analysis into this subject and dismiss various wives tales along the way.
Oberfell & Thompson tested five different shells out of the same gun and found an 11% variation in patterns (70-81% at 40 yards). So the shell in the chamber is as relevant as the constriction at the end of the barrel when thinking about this stuff.
