Meh, it's all in your particular choice of terminology, and even choice of vernacular, as the terms are not explicit, and have different meanings in the common, legal, and engineering senses. To be explicit: Machine gun: In common parlance, it means a fully automatic firearm of any type, including submachineguns such as the MAC/Ingram/UZI, light machine guns such as the M16, AK47, FN-FAL, and even heavy belt-fed machine guns such as the Browning M2. The legal definintion is almost the same, and includes light machineguns and heavy machineguns under the simple heading 'machine gun.' That is to say that on the registration paperwork for a fully-automatic Browning belt fed or for an AK-47, the firearm type is called out as, simply, 'Machine Gun.' Technically, the term machine gun is broken into the three classes outline above. Submachinegun: Technically, a submachinegun refers to a pistol caliber machinegun, i.e, the MAC, Uzi, Thompson, etc. The legal definition appears to be the same. In common parlance it almost always refers to the Thompson Submachinegun. Automatic Rifle and Automatic Pistol The two most nebulous of phrases in the firearms world. Strictly, they refer to semi-automatic rifles and pistols, respectively, and not necessarily those capable of fully automatic fire. A better equivalent phrase is 'self-loading' rifle or pistol. The difficulty arose from the fact that automatic-loading firearms were developed prior to fully-automatic firearms, so sem-autos were simply called 'automatics' back in the day. In common parlance, the phrases mean pretty much anything and are thus worthless, technical people will avoid their use outside of the community. Assault Rifle Strictly, an Assault Rifle is a fairly light, very durable, reliable and manuverable infantry rifle that is capable of select-fire (choice of semi- or fully-automatic fire modes). Common examples are the AK-47, the M16, and the Sturmgewehr. Commonly, an Assault Rifle is anything that looks marshal and can fire semi-automatically. In other words, calling an AK a 'machine gun' is not strictly wrong, and in fact legally correct, but is still a poor choice of words. As is 'Automatic Rifle.' I'd choose 'select-fire assault rifle' if I had to pick. As for the 'engineering difference' between a self-loading rifle and a fully-automatic rifle, I feel confident in saying, as an engineer, that in many cases it's not much. Regards, Huntsman