While I'm sure there are many others, several places come to mind:
Simmons Guns in Kansas is noted for top quality shotgun work.
http://www.simmonsguns.com/
Hans Vang is FAMOUS for his shotgun work:
https://www.vangcomp.com/A good one in Missouri is Nu-line Guns. They USED to be in St Charles, but it looks like they moved:
www.nulineguns.com/Although they are listed as one of American Handgunner Magazine Top 100 gunsmiths, seems they are more oriented to shotgun work now.