Yes, if you a attach a stock to a handgun it will become a SBR.
It doesn't matter if it is single action.
The only thing that would get you exempted from this is if the gun is black powder/muzzle loader or if it is a curio/relic gun that was originally designed to use a stock and is specifically expedited from NFA status.
"No, it's not a silencer. This little doodad is my own invention. I call it a loudener." --Sledge Hammer