If you look at the radius in the corners of the fire control pocket. a 7/16" end mill actually is the perfect tool for the correct radius. Works great.
+1 for McMaster. They have a huge selection of metalworking tools, parts, and stock. Plus everything they sell is legitimate and not fake brand copies, which has not been the case for purchases of mine from some of those other online markets mentioned.
I'd rather pay a little more for a real US or European-origin bit made with known metallurgy and dimensional QC, rather than something rough machined out of a probably former railroad tie plate in a third world country.
McMaster Carr mentioned above and MSC Industrial http://www.mscdirect.com/ProductsHomeView were the common sources manufacturing companies use for small quantities of tools and supplies when I was working.