There are a lot of common misconceptions about both the AR and AK platforms.
First of all, the AK is not an inaccurate weapon as a lot of people seem to believe. It is plenty accurate out to 300m, which is a distance far greater than what you're likely to be seeing in game. It may not be the flattest shooting round in the world, but at the ranges you're likely to be shooting in game it should more or less hit where you're aiming with little deviation, they're probably a 2MOA gun, and for a standard issue rifle that's perfectly accurate enough. Secondly, the 7.62 isn't as disgustingly hard to control as many people will lead you to believe. Although I have my doubts about the quality of training the NVA would have received, it's completely possible to hold the rifle on target for an extended burst unless you're built like a twig.
And about the damage debate, people seem to have the misconception that because the 7.62x39 is a larger round it is more "damaging". Although as someone previously stated, it has a massive advantage when it comes to vegetation and light barrier penetration, the round itself was nothing special when it came down to its design. It is a classic FMJ, it hits the target and goes through it; that's a lot of lost potential for energy transfer. On the other hand, while the 5.56 may be a smaller round, the round was designed very carefully and was quite revolutionary with its terminal ballistics. The 5.56 was designed to tumble and fragment upon impact with a soft target, ensuring all the energy behind the round went into the target rather than using most of it to pass through. Out of a 20 inch barrel you're looking at a fragmentation range of roughly 150m not accounting for inconsistencies in velocity from cartridge to cartridge, and also not accounting for losing velocity while penetrating through vegetation or light barriers which was a known issue during its use in Vietnam. The guaranteed energy transfer of a fragmenting round (in the right conditions, of course) alongside the fragments having a greater chance to strike vitals that an FMJ on a straight path through may not have hit makes the 5.56 an absolutely devastating round up to its intended ranges.
That being said, if an XM-177 is introduced like I'm hoping we will hopefully see a realistic difference in fragmentation range between the M16A1 and its carbine counterpart. Having an 11.5in barrel versus the 20 inch barrel of the M16A1 makes a massive impact on the velocity it achieves before it leaves the barrel, and drastically reduces the range at which the round is going to fragment.
tl;dr the differences between the two will mostly be apparently when firing through vegetation and when engaging at extreme distances, hopefully this will be modeled correctly.
