Concur on law - but, I trhink the definition used in law was purposefully made to make non-machine guns subject to machinegun restrictions.
Except that there were no federal restrictions on machine guns prior to the National Firearms Act of 1934. Not on BARs, nor M1917s, nor M1919s nor M2, nor M1928 submachine guns.
The law was motivated, supposedly, by the misuse of sawed off shotguns and submachine guns by the likes of Machine gun Kelly and Al Capone's boys. Of course by the time they got it passed, the motivation for all that, prohibition, had been repealed. There were folks like Bonnie and Clyde, running about with BARs and the civilian version the Colt Monitor. (although even B&C had met their end, via those same sort of short shotguns, BARs and Colt Monitors, about a month before Congress passed the NFA. The law still resides in the Internal Revenue part of the US Code, even though they no longer collect the tax on machine guns. (They still do on the other weapons covered by the NFA.