A 2005 ATF letter which addresses the rear stock tang on AKs
Summary
C&R Collectors not prohibited from building guns for personal use. Markings not required unless transferred.
Back plate on a sheet metal receiver (commonly called a "rear trunion" by the online gun community) is not a trunion per ATF. Trunions are barrel supports.
Guns may be built using imported barrels from parts kits.
A foreign-made barrel with a permanently attached US-made muzzle device is still an imported part.
List of counted AK-47 parts for 922(r) purposes.
Reiteration that the back plate is not a rear trunion in a stamped sheet metal build.