first off its further than 300 meters try 500 meters on point target and 800m(iron sights) on an area target the only difference between the two in my opinion is roles what role do you want to play do you want to be fighting house to house with an ar15 maybe but more important do you want to be 500+ meters out with an ak when your enemy has an ar15 and its not that the aks arent designed to go the difference its that the round 7.62 is literally too heavy and just drops out of the sky and thats why they have a bad rap at longer distances. the conscripts werent untrained just poorly equiped for the role they were trying to pull off... watch videos from africa when this crazy drugged out mindwashed "freedom fighter" fills the air with led and next thing you know he is getting picked off from some crazy ass distance by a marine with his m-4/m-16 variant. how else do you think the delta force pertrade in black hawk down was able to hold off the malitia for so long.... they had a team each of them had their own role and knew the limits of their enemies and thats what made them bad asses not the weapons...
sry ive always hear 300m. and yes MOST conscripts and freedom fighters have limited if not no training. 7.62mm does in fact go further than .223. Heavier bullets go the farthest, Thats y most long range sniper rifles are large caliber weapons (.50, .408, .416, ect.) i know there are other factors in that but thats not the point. so 7.62x39 has the capability to go farther but it wont be very accurate. and Delta force did such a beast job @ black hawk down because they are elite and the militia were not properly trained. its like putting a MMA fighter against a middle school wrestler...
Most long range sniper rifles are not chambered in those calibers(except in stupid fucking video games) being that the .50 etc are actually anti materiel rifles. Most systems are chambered in and around .30 cal. The .50 is used in a relatively limited number in our military as compared to the .30 cal M24, M40, M110, etc used by our forces. That is primarily due to portability of the platform, ammunition and cost.
On topic, the AR is a superior platform due to accuracy of the round. The USMC has always trained their riflemen, ie. all hands, to effectively engage targets out to 500m with iron sights. I own, and shoot often, both platforms and am a pretty good shot, the AR wins out hands down each time. The modularity and adaptability of the AR also make it a winner over the AK. Availability of components world wide is fine for either. I don't plan on being in the middle east, Africa, or any other fucked up part of the world when the zombies attack anyhow.