The only thing I have issue with correspondence schools for gunsmithing is the lack of hands on training. Watching examples on video or online, etc. like the courses advertised by the Gun Club of America, etc., may be good for theory, but for practical experience, I don't know.
There's one of the best schools right in my backyard. They've been around since 1949. I just wish I had the time and money to enroll.
http://www.pagunsmith.edu/about.php