FYI there are rifle conversions made for many pistols, including the mark II, glocks, 1911's, beretta neos ect. I have found no information stating it is illegal to add a shoulder stock to a handgun as long as the barrel length is a legal rifle length. Adding the shoulder stock to a handgun with a barrel length of less than 16" would be illegal as if i'm not mistaken would categorize it as a SBR or AOW. Maybe I am mistaken but if it requires the forms and tax stamp I wonder how companies market all of the above conversions and uberti produced 18" buntline cattleman SA revolvers with detachable shoulder stocks and no extra taxes or forms?
Pretty much the same issue with T/C contenders. You can have an 8-14" pistol that with readily accept a factory made stock for the unit that would make it a class 3 weapon if it were to be installed on a frame with a barrel less than 16". But as long as you have a 16"+ barrel on the frame when the shoulder stock is in place you're A. O. K. Maybe i'm missing something?