You absolutely can SBR a pistol.. as Dallassb said it's common to take a pistol configuration, SBR it and mount a stock.. the Draco pistols, Kel-Tec PLR16, HK mp5(k) pistols, etc.. they startout with short barrels, people SBR them and then add the stock.. no problemo.
for non NFA, I understand the law exactly as carguychris said.
If it's a rifle it stays a rifle, you'll need SBR to shorten in.
guns that start out as pistols can be converted to rifle configurations and then back again into pistols.. although this in no way allows you to add a stock to a short barrel (SBR)
But you can swap a longer barrel then add the stock and it's in rifle config, no problem, then reverse that to go back to pistol.. again this is only for combo guns and guns that are sold as pistols.. anything sold as a rifle must stay a rifle.
Im actually in the middle of dealing with this my self on a AR15 build.. I know I should build it out as a pistol first before configuring as a rifle but I have no idea how to document (without a doubt) as a pistol "1st" build.
Im building from stripped lowers which aren't sold as rifle or pistol on the dealer forms so it's on me to somehow document the 1st build.
@Deja vu
Certain old C&R (curio & relic) firearms are exempted.
First thing that pops into my mind is the old c96 broom handle.
It was originally made with a shoulder stock, the gun could actually fit inside the stock for storage.
Unless something has changed THOSE CAN be fitted with a stock (I think stock reproductions of the same design are also allowed but don't quote me) and are legit legal without a tax stamp.
I know a lot of times the stocks are missing, cracked, damaged on the c96's
They were made of wood iirc
Like to have one of the Red 9's (9mm chambering)