DB, you can also try getting Art DeCamp's book on all the screwtip styles that should be on the KRA website, and Art goes to great lengths to teach the students about all the detail characteristics on each style/school. Valuable.
In addition, for constructing a screwtip powder horn try the Book of Buckskinning Vol II, although it is for a York county style screwtip.
For 'internal' style screwtip like Tim has done here, I believe there is a tutorial or two on this site that addresses the internal screwtip construction.
Gary