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An AR receiver is aluminum, that's why it won't rust like an AK. The AR is "cast" aluminum, that’s why the receiver won't get bent or damaged as easy as the .8mm or so stamped AK. The magazine loading is not as easy, the soldier on average for weight and volume carries less ammo, and the gas tube design on the AK lends itself to easy damage. The aluminum is lighter than the steel as well on the AR. However, the AR15 from the day of its inception was more expensive because of the cast aluminum specifically, but also tighter tollerances, and chrome work. The 223, is a flatter shooting round and from 0-300 meters and has less deviation from line of sight (Above and bellow). With less recoil especially because of the buffer spring design the AR is easier to manage on auto or burst, with a closer grouping when firing multiple shots. The AK was a cheap and easy to mass produce weapon for countries with very limited industrial capacity and limited money. It's a "cult gun" and although very reliable (although not indestructible as some believe), this weapon NEVER was an exceptional good design in those aspects that would make it accurate, manageable, light.....etc. In fact the 7.62 round with it’s nice stability (18 cm penetration before turn) tends to punch nice clean holes through, while 5.56 already rotates at 7 cm with higher velocity (ripping apart non-elastic organs better) causing in human like targets more damage. The AK is heavier, less accurate, more corrosion susceptible, less manageable and frankly not even more reliable, and on average with even less target effect on a human. The rest of the world (France, Germany, UK, Austria, Australia, Korea….etc) in the meantime has gone to a high velocity smaller round, including the Russian 5.45.
The AR costs more; it’s also a lot more gun. If you like your AK, I'm happy for you, but it's a cheap gun for a reason.