The original 1934 National Firearms Act excluded .22 rimfire weapons from the definition of regulated arms. There is a clause that states this quite plainly. A .22 rimfire was not considered a weapon and not subject to federal regulation or taxation. I wonder if this was amended subsequently or just ignored by the ATF? If they decided, for example, that a .22 rimfire machine gun needed to be regulated and taxed, then they either had to amend the firearms act or reclassify them as "AOW"s, which would make them subject to a five dollar tax rather than the $200.00 tax for real machineguns. It would also exclude them from the 89 "Firearms Owner's Protection Act" that banned the future import or manufacture of automatic weapons for civilian markets.