You find the .410 offered i pistol form because it is close in measure to the .45 colt. The .410/.45 colt bbls have rifling in them, thus making them legal and not in violation of the GCA. Since any other shotshell is nowheres near compatabile with a centerfire rifle or pistol cartridge, and making a smoothbore shotgun pistol would be illegal you will not find a pistol length shotgun bbl (18" is the shortest a smoothbore shotgun bbl can be.)
Brian M.