Magpul AR-15 stocks and Rifle Scopes

What are NFA Weapons?
There are two kinds of firearms under U.S. (federal) law, title 1 firearms and title 2. Title 1 firearms are long guns (rifles and shotguns), handguns, firearm frames or receivers, and most NFA weapons are also title 1 firearms. Title 2 weapons are NFA weapons. Title 2 of the 1968 Gun Control Act is the National Firearms Act (26 USC sec. 5801 et seq.), hence NFA. Title 1 is generally called the Gun Control Act, (18 USC sec. 921 et seq.). NFA weapons are sometimes called class 3 weapons, because a class 3 SOT (see below) is needed to deal in NFA weapons.

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[see 18 USC sec. 923(a), also ATF Form 7 and Form 7CR]