This thread. Post #19.
http://thefiringline.com/forums/showthread.php?t=416934
The case you were directed to is specific to the Thompson Contender KITS. Per the Supreme Court ruling, the THOMPSON CONTENDER KITS, when bought in ONE KIT can be used to configure any weapon you choose. Rifle to pistol and pistol to rifle, as you choose.
YOU CAN NOT do this with a your glock or your 1911.
Once a Rilfe, ALWAYS a rifle. Unless you file a form 1 to make a SBR.