This is a real gray area.
**Disclaimer: the following information is my interpretation from trying to make sense of this, and may have no real solid foundation in law.**
As far as I know, rifle buffer tubes can be used in pistol builds.
The way I see it, you can choose to use a carbine length buffer tube, you are good to go, since a carbine buffer tube isn't meant to be fired from the shoulder. If you choose to use a rifle length buffer tube, you are good to go since a rifle buffer tube alone isn't meant to be fired from the shoulder. If you take a carbine length buffer tube and put something on it to make it longer, you are walking on eggshells. You could argue that it isn't meant to be fired from the shoulder, but some JBT ATF agent might argue that you have made it into a device that is meant to be fired from the shoulder, since you unnecessarily made that thing that sticks out from the back of your firearm longer. YMMV.
My suggestion is to just put a pistol buffer tube on it and submit the paperwork to make it an SBR if you are concerned about shouldering your firearm.