Do I have to use a dedicated pistol buffer tube on my AR pistol build?

The ATF has issued an opinion letter stating that the use of a 6 position carbine buffer tube is perfectly legal for use on an AR pistol in spite of the fact that it can readily accept a stock. The only way you can actually run afoul of the NFA is by installing a stock on the buffer tube creating an unregistered SBR. The existence of the carbine buffer tube by itself does not make the weapon and unregistered SBR but to avoid the issue altogether it is advisable to use a dedicated pistol tube.

big bore