The national firearms act of 1934 classified any firearm with a buttstock and either a rifled barrel under 16" long or an overall length under 26" as a short barreled rifle, and requires a $200 tax stamp. (see http://en.wikipedia.org/wiki/National_Firearms_Act)
The acquisition of this tax stamp is an undue hardship to law abiding citizens. It makes a legally purchased pistol no more destructive to add a butt stock or fore grip to the frame. Furthermore this provision discriminates against citizens that have disabilities, and who also wish to legally purchase a pistol caliber firearm by not allowing the individual to decide whether or not to attach a butt stock to the firearm in question.