MP5s are THE sub-machine guns. They are in no way "assault rifles". Besides, what constitutes an ASSAULT rifle? I know there's carbines...and rifles....and sub-machine guns...and machine pistols, however, no assault rifles.
AR does not mean assault rifle.
Assualt rifles are like the midsize SUV, like my Grand Cherokee... the market is VERY broad and they are bigger then the small SUV, like your liberty (carbine) and smaller then the big boys, the Yukon and excursions (LMGs). all bigger then the honda civic (SMG) with the 4 cyl, which is bigger then the Geo Metro 3 banger(the machine pistol)... yes the 2 door spec had a 3 banger...
the way I understand, and this is ONLY how I understand it.. it is the difference in size of firearm and caliber. the generic term rifles generally describes a single action, usually bolt action and many of hunting type calibers. thier accuracy focus in around on a single shot. I see it as assualt rifles, or battle rifles, have certain calibers, usually higher velocity loads for human targets to be specific (I.E. .223 and .308) and select fire or semi auto trigger groups.. assault rifles are battle rifles, carbines being a shorter battle rifle with the same load and SMGs were usually handgun ammo and even smaller barrel lengths then a carbine. machine pistols are even smaller usually with out a stock. that is just how I usually break them down I could be very wrong and willing to admit it if I am.
AR stands for ArmaLite