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.
It's worth knowing that the G3 has 17 ATF-counted parts - three do not apply to the G3-series rifles. So, for folks who insist on only half-understanding 922(r), the G3 needs 7.
This list is by no means authoritative, but it does look plausible to me. I pulled it from an amalgam of posts I read on hkpro.com and a few other internet forums, however I would very much like to see an official ATF answer on what constitutes counted parts on the G3 rifle. I believe the CETME rifle has the same overall design and will have the same parts count, but I couldn't find any specific CETME information out there.
Checklist for 922(r) compliance on G3/CETME builds
Please note that the as-imported configurations are compliant. This checklist becomes important whenever you wish to replace original parts on an imported rifle, or if you are building one from the ground up.
-- SeanNewton - 10 Dec 2008