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When in doubt, check with the ATF. The above is a quote from the ATF Online Learning Guide http://www.atf.gov/firearms/ffrrg/index.htm(M25) If a person has a pistol and an attachable shoulder stock, does this
constitute possession of an NFA firearm?
Yes, unless the barrel of the pistol is at least 16 inches in length
(and the overall length of the firearm with stock attached is at least 26
inches). However, certain stocked handguns, such as original semiautomatic
Mauser "Broomhandles" and Lugers, have been removed from the purview of the
NFA as collectors' items.