You need to read ATF rules (and even the applicable Federal Code and NFA) very carefully. In many cases when they say 'firearm' they mean an NFA weapon.
The BATFE web site has the most common answers, but you still need to watch what part of the site you are reading.
(A7) Does the GCA prohibit anyone from making a handgun, shotgun or rifle? [Back]
With certain exceptions a firearm may be made by a nonlicensee provided it is not for sale and the maker is not prohibited from possessing firearms.
However, a person is prohibited from making a semiautomatic assault weapon or assembling a nonsporting semiautomatic rifle or nonsporting shotgun from
imported parts. In addition, the making of an NFA firearm requires a tax payment and approval by ATF. An application to make a machinegun will not be
approved unless documentation is submitted showing that the firearm is being made for a federal or state agency. [18 U. S. C. 922( o), (r), (v), and 923, 27 CFR 178.39, 178.40, 178.41 and 179.105]
http://www.atf.gov/firearms/faq/faq2.htm#a7