I like a BC gap to be between .003 and .006. Less than .003 is too tight for my liking (possible lead jamming) and over .006 will result in excess velocity loss and possible flame & lead spitting.

Smith and Wesson will CURRENTLY accept up to a .012 B/C gap which in my book is a way too much - but they get to say "that's normal" instead of fixing the larger ones.