See also: Sporting vs Non-Sporting Firearms
The United States has a lovely bit of import law, Title 18 USC § 922(r), which prevents most of the more interesting rifle designs from being directly imported into the US. The import regulations are commonly referred to as "922(r)" in gun circles.
In order to legally build a non-sporting rifle out of a parts kit (or modify a 'sporting' rifle to include 'non-sporting' features), no more than 10 imported parts may remain in the final product. This is generally done by replacing foreign-made parts with domestically-built parts.
Title 18 USC § 922(r) - Prohibition against domestic assembly of weapons banned from importation as non-sporting
Exceptions to the Requirement to Comply with 922(r)
The ATF seems unable to make up their mind on whether or not 922(r) applies to NFA weapons. In 1994, they issued this letter, which states that NFA weapons are not required to comply with 922(r). However, 15 years later they issued this letter, which states that NFA weapons enjoy no such exemption.
This creates the unfortunate position that an earlier letter says that NFA items must comply, and a later letter says that they aren't required to.
There are two rational ways and one irrational way to view this situation:
The later letter should take precedent, and therefore NFA items must comply with 922(r).
The letters contradict each other and may both be safely ignored. Therefore, one must simply rely upon the wording of 922(r) - and thus one must comply with 922(r).
The 1994 letter, because it came first, is binding and somehow the 2009 letter is not. Therefore, NFA items are not required to comply with 922(r). This is the irrational view - either of the two above are reasonable, and both lead to the same conclusion.
922(r), magazines, and constructive possession
TheDrickel, of Calguns, has pointed out to me a very favorable letter from BATFE. The letter in question is scanned in as two pages (page 1 and page 2). In this letter, it is stated that constructive possession does not apply to magazines, when using magazine parts in order to achieve 922(r) compliance. This is very nice, because conventional wisdom in the past has been that if your rifle needs US-made magazine parts in order to be 922(r) compliant, that you had to convert ALL your magazines to have the same number of domestically built parts in order for it to count.
Generic 922(r) Checklist
Note: This checklist is generic, and contains all the parts which 922(r) is concerned with. TGW has specific checklists for certain types of rifles. If you happen to have one of those rifles, you should consult the rifle-specific checklist instead of this generic one.
The specific pages are:
The sum of all the check boxes in the form below will automatically add themselves up as you click and unclick them. This allows you to easily consider how to bring your rifle into 922(r) compliance with non-sporting features by providing you with a component count. Bear in mind that not all rifles actually have all of the parts on this list - for instance, the AK-47 combines the operating rod and gas piston into one component. If your goal is to render a rifle 922(r) compliant, it is essential that you have an ATF-approved list of the counted parts for the rifle in question.
The list of parts which 922(r) counts is:
-- SeanNewton - 17 Jun 2007