From the ATF website -
https://www.atf.gov/firearms/curios-relics
(Posted/linked here on 19 Sept 2017).
I expect we are mostly interested in the exemptions, Section III, of the list.
Questions always arise as to what buttstock is legal; if carbines with barrels less than 16" are legal; if reproduction buttstocks are legal, exemption for Benke-Thiemann folding shoulder stock, etc.
This should serve as a general guide.
