Wikipedia may be helpful here: http://en.wikipedia.org/wiki/Assault_Weapons_Ban#Definition_of_assault_weapon

A true 'assault rifle' is a * Carbine sized individual weapon ( note 'carbine' it is shorter than a rifle, with a barrel of around 20" or less) * Capable of selective fire. Semi- or fully automatic * Fires from a locked breech. You don't have to 'cock' it. * Utilizes an intermediate powered-cartridge. More powerful than a handgun, less than a "battle rifle." Comparable to many 'hunting' rifles. * And has a 'large' capacity detachable box magazine. From 20 to 50 or even more rounds. The 'assault rifles' you hear about on the news when there is some news worthy shooting are not assault rifles. They are semi-automatic, they fire one shot every time the trigger is pulled, not fully automatic like a 'machine-gun.' They have a permanently mounted magazine of limited size or some provision that prohibits mounting a larger one. You will also hear 'high powered' assault rifle, another mis-use of the term. Of course a semi-skilled gunsmith can modify the civilan versions to increase the magazine size or make them fully automatic, especially on the cheaper weapons. This makes the weapon more dangerous to the user, sometimes more than to the victims. The cheaper ones are inheritantly less safe anyway.

The definition of an assault rifle is that the rifle be of a non pistol caliber, able to shoot on semi, or full automatic setting. having a shell of an intermidiate caliber, the assault rifle came out of the ashes of world war two, where as the german, british or canadians all used a bolts action type rifles as their main infantry weapon, and the americans had a semi auto m-1 gurrand which shot a 30-06 to 30.cal there were others but there main machine gun was the browning bar and all the allies had the tommy, do you see a pattern, look at how many different guns all with different ammo. The germans forseen this and they came out with the worlds first fuctioning assault rife and was known as the MP-43 or Strumgrewer and had the 8mm bullit with a smaller casing this gun came to late but it could shoot farther than any submachine gun and it had a bannana 30rd clip. The weapon was first copied by the russian that took the same relative design and and came out with the conviently timed AK-47 that looks like the MP-43 that the russians should be ashamed of such a blatent theft of technology, like you give them a design for a spoon and they say that they invented it, its frustrating, any ways the americans had their M1-gurrand and it is a great gun, but they wanted to get a way from that design because of it's shell ejection that made a very sudibl ping signifying your empty, so they went for the M-14 that had a fully automatic with semi automatic option could handle 30rd clip but the rifle still housed a heavy caliber bullit .308 and it was not pratical so it was discontinued and had the shortest run as any infantry weapon every use by the US military, in 1965 the US got the M-16 which house the 5.56 got the jamming prolem and cleaning prolems and regulary mainteanance taking care of and the M-16 is still in the field. But you will find the genisis of the Assault Rifle date from the german MP-43 to now with the most numerous weapon in the history of man the AK-47 to our M-16