Because the definition of rifle includes "a gun made from a rifle" and a short barreled rifle is a subset of rifles with certain length characteristics.
Lets see if I remember this correctly (note most definitions are rercursive)
MG- anything that fires >1 shot per trigger operation (and more depending on whether akins accelerrator ruling gets overturned), plus things that turn semiautos into MGs, plus semiautos that are easy to turn into MGs, plus things that are required for reactivating MGs (not the law but the ATF interprets it this way for M16 auto sears), plus certain weapon designs that are like semiauto designs that are easyto turn into MGs. Enormously complicated even without the scienter stuff that got added in 1994.
Shotgun- shoulder fired firearm for discharging shot or gun made from shotgun
Rifle- shoulder fired firearm or firearm made from rifle
Pistol- meant to be fired from one hand
SBR- rifle + measurement elements
SBS- shotgun + measurement elements
DD- weapon that takes explosive/grenade/mortar ammo, rifled weapon > .50 caliber (including pistols), non-shotgun weapons (not signal devices) > 50 calibers, shotguns that look scary
AOW- pistol without rifling, short barreled single shot combo guns like game getters, things that can be concealed and arent in the other categories like belt buckle pistols, cell phone guns, pen guns, wallet guns (but not holsters)
Note the incredible level of nuance here. Not all firearms are necessarily weapons, some are signaling devices. Some shotguns are smoothbore pistols, some are DDs, some are SBSs. Anything that is an MG is in no other category. If it wasnt for the 86 ban, nearly every SBS and SBR registration in the past 20 years would probably be an MG registration instead. Especially all those AR/AK SBRs.
