Michael Shurkin, a political scientist at RAND, offers a brief history of the assault rifle, with this conclusion:
Virtually all the world's armies now use assault rifles, most of which are variants of the AK-47 or the AR-15. They differ in the details--slightly smaller bullets or slightly bigger ones, longer or shorter barrels, and so forth--which reflect different schools of thought regarding the right sweet spot between power and ease, between full-size rifle cartridges and pistol ammunition. There are also different mechanical approaches to things like how the gun uses the gas from a fired round to reload. The basic idea, though, has remained the same since Hitler gave the weapon its name. Other guns are technically more lethal, and of course different guns are better suited for different purposes. Assault rifles were designed for fighting wars.