ATF didn't Officially Define Shoulder Stocked
Pistols as "Short Barreled Rifles" until 1961
Image from Rock Island arsenal
By Dean Weingarten. Sept 28, 2021
A proposed rule change about arm braces on pistols prompted this correspondent to research the history of shoulder stocks, pistols and the National Firearms Act (NFA).
When the National Firearms Act of 1934 was passed, pistols with shoulder stocks were not mentioned. Short barreled rifles were not intended to be in the law. They were inserted at the insistence of a confused congressman on the House Ways and Means Committee.
Fitting a pistol with a shoulder stock had always been an option. The 1934 NFA was imagined to be regulating shotguns which had been shortened to create concealable, awkward pistols, not pistols which had a stock added to them, creating a carbine more accurate, but less concealable than a regular pistol.
The primary targets of the 1934 bill had been pistols, revolvers, sawed off shotguns, silencers and machine guns. Through lobbying by the NRA and concerned citizens all over the country, pistols and revolvers were taken out of the bill.
..... 
"The National Firearms Act (NFA) was a botched disaster from the beginning. Reading the transcripts of the Ways and Means Committee hearings is like a re-run of gun control hearings of the today. The same mix of arrogance and ignorance is displayed."
