One thing that I have learned about recently is the misinformation due to kill cam lag. The guy holding the sniper rifle has a completely different view of the kill than what they killcam shows (which usually only shows a half scoped in shot. This is just killcam lag and causes most of the frustration from people. If you do not believe me, here is a link to a video that shows you side by side, the snipers view compared to the killcam. QUICKSCOPING = CHEATING?! Call of Duty Ghost "Quickscoping Gameplay" Multiplayer Gameplay "Xbox One" - YouTube. The guys making it seem easy have most likely been doing it for years, but take a look at that video and tell me the comparison on those viewpoints doesn't change your opinion on how cheap quickscoping is.
Like stated above, using an AR or the m27 can completely mitigate snipers. If they don't get the drop on you and catch the first shot (which doesn't even always hit), hipfire can completely nullify a sniper. all you have to do is land a bullet or two to knock off their aim, even when they are running the focus perk.
To be fair too, If you are quickscoped, the only difference between that and going head to head with someone using an AR, is most likely just a hit marker or two. First shot=win in this game.