I got several questions about this today, and I thought I'd clarify.  As of November, you must be 21 years old to receive a frame or receiver for a firearm.  It doesn't matter if it's to be used to make a rifle or a handgun; it's now treated in a similar manner as a handgun.

Congress didn't pass a law while we weren't looking, nor did the ATF sneak a regulation in under the radar.  This is simply a clarification and enforcement of a clause buried in Code section 922, which states:

[It shall be unlawful for any licensed importer,

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