[PTK]

This is in reply to your correspondence which was received by the Firearms Technology Branch, Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF), on January 30, 2008. In your letter you inquire about the attachment of a folding stock to a pistol having a barrel length of 16 inches or greater.

As background, 27 CFR Sec. 479.11 (Meaning of Terms) states, in part:

The overall length of a weapon made from a shotgun or rifle is the distance between the extreme ends of the weapon measured along a line parallel to the center line of the bore.

Based on this description of overall length and its correct measurement, ATF has taken the position that firearms having folding or collapsible stocks are properly measured for overall length with the stock fully extended.

In the situation you present, the attachment of a folding shoulder stock to a pistol having a barrel length of 16 inches or greater would be lawful as long as the overall length of the resulting firearm is at least 26 inches with the stock fully extended. We caution that, because the configuration you have specified results in the manufacture of a rifle, a subsequent reconfiguration of the firearm to a pistol configuration would result in a weapon made from a rifle, which is a weapon controlled by the National Firearms Act (NFA).

We thank you for your inquiry and trust that the foregoing has been responsive.

Sincerely yours,

John R. Spencer
Chief, Firearms Technology Branch

PDF of 27 CFR Sec. 479.1. Search for "extreme ends" and you'll find the relevant passage.