The National Firearms Act of 1934, as it originally was written, was intended to ban all concealable firearms and the wording of the law included handguns. This is where the "Any Other Weapon" category comes in (as in "any other weapon or device capable of being concealed on the person") and why short-barreled rifles and shotguns are included in the NFA.
Successful eleventh-hour lobbying by the National Revolver Association got handguns stricken from NFA '34, which has resulted in eighty years of silliness where it's illegal to put a wire shoulder stock on a .22 derringer but my .243 Winchester deer-hunting pistol is okey-dokey. Also this.
(You want a reasonable short-term play to move the RKBA ball downfield? Get SBRs and SBSs moved from Title II to Title I.)