Just my two cents, fwiw...
Campaign / Legendary:
1st two elite kills were melee. Several grunts in various level locations while sprinting. I noticed no discernable issues with hit detection as it relates to melee.
With the BR, I was a bit frustrated but with distance comes spread. I may be zoomed in, but when I pop the trigger, I'm still as far away as I was before I zoomed, if you get my drift. Ranged spread seems to be what I would expect. Close quarter gunfights seemed mostly accurate as well. Whether or not hit detection was designed to be more technical I do not know. I could be making something out of nothing, but so far it feels as it should imho.
Disclaimer: I have not played mp yet, but there are a myriad of things to consider: LAN/Wireless, Internet/Lag, Server/Load, HDD/Optical latency if loading data. Then, there are times I simply can't hit the broad side of a barn in campagin. I'm sure it will be much worse when I start mp! lol