922(r) Worksheet for MP5 builds and Various Clones
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 MP5 has 16 ATF-counted parts - 4 do not apply to the MP5-series rifles. So, for folks who insist on only half-understanding 922(r), the MP5 needs 6 US-made parts to be compliant.
This information is derived from an ATF letter contained in the back of the owner's manual of my Coharie Arms CA-94 9mm MP5 clone. The letter was dated July 9th, 1998 and was apparently signed by Edward M. Owen, Jr, chief of the ATF Firearms Technology branch.
-- SeanNewton - 10 Feb 2009