If you use a 16 to 20 Gaugemate, you will get a significant pressure drop soon as it leaves the chamber with minimal loss of fps. It has been my experience, when using 12 to 20, that the pattern size is about 1-2" larger.

http://www.doublegunshop.com/forums/ubbthreads.php?ubb=showflat&Number=234173&page=2