AR's do not act like SR's in this game, even teh SR does not act like a true SR for there is no bullet drop in this game. AR's do not win every battle all the time, if they do against you then clearly it is your tactics and aim and reaction times that are the problem when you use other weapons.
As toward firing behind someone and still getting the kill, that was lag and desync, had nothing to to with hit detection not working correctly. Lag will also cause you to hit players that have moved on, not just having to pre fire ahead of them.
There are no lmg on bipods in this game. You never deploy the bipod in this game on any of the weapons ever. Even if they have one so that is a moot point.
The AR will always be the goto weapon used for all situations with fair to good results; but in the right hands the other weapons can easily outclass the AR's in their intended ranges. So no the AR's are by no means at all OP.
And no weapon in CoD ever was OP, just players like to cry about any weapon that beats them and their chosen weapon at all.