The Bailey & Nie "English Provincial Gunmakers" lists everyone they could find except the London makers. Most of them were in the B'ham area but the 2nd half of the book lists them by county and town. Along with Blackmore's work on the London makers, I think these are the only two general reference works that are consistently reliable...
That said, I doubt much real "gunmaking" was going on after 1800 and practically none by 1850. B'ham so thoroughly dominated the trade that, at best, other provincial gunmakers were buying in parts and assembling them. By 1850 the Provincial gunmakers were repair men that retailed B'ham products, often marked with their own names.