People jump on this topic for two reasons.
The first is because they took the time to learn to quickscope, and create a situation where even though sniper rifles are niche weapons, they can be used somewhat effectively in short to medium range situations (read: every map in MW2/Black Ops), by simply bypassing the range issue. This strategy is currently defunct in the new CoD because they changed the game to misfire your weapon when you scope. They broke the games bullet physics to gimp a playstyle. This is why quickscopers are upset.
Everyone else is upset because they got their asses whooped by quickscopers in other games, because they don't know how to deal with them. The argument? More realism! It's more like real life, where you can't snipe after just turning a corner! The only flaw in this argument is that MW2 isn't realistic at all... As a matter of fact, there isn't a realistic fps out there, it'd be retarded.
First of all, weapon stats are completely broken. There are guns in MW2 that are unbelievably powerful and had to be scaled down to not kick everyones ass. The TAR 21 is unbelievably accurate in real life, up to hundreds of yards away. It was gimped because it needed to be.
My point isn't that quickscoping is or isn't a legitimate strategy (although it really isn't a big deal to handle). I'm stating that bringing realism into the picture is asinine since realism is ALWAYS thrown out the window whenever video games become involved. Realism is broken and toyed with and manipulated until it becomes fun. Thus, a game where you can instantly scope and murder someone, or... not in this case.
TL;DR stfu about game mechanics that are changed for the idea of balanced gameplay. If you don't like it, learn a different way to play... that's what I did in MW2 to murder quickscopers anyway.
Edit: Why is it that everyone on this forum disses shotguns? I've used FMJ and Foregrip on the SPAS-12 and murdered people at 40+ yards away on HC mode and nobody bitches about how I rape them in one shot half a football field away but you're all crying about how unfair it is that someone who is ridiculously accurate and precise (with the exception of auto aim, not included in PC) can pop around a corner and merc your ass with a scoped weapon? Fucking hilarious.
Double Edit: I'm actually still not done ranting. Every gun class was rigged in MW2, there's no reason to come down hard on people for sniping. Assault Rifles had the ACR, which practically fired fucking laser beams, and the SCAR, which punched holes in everything to devastating effect. LMGs had the RPD which has more stopping power than a train and the AUG which has almost no recoil for a machine gun. Shottys had more rigged akimbo bullshit than you can shake a stick at, let alone extended mags AA-12 shenanigans. With pistols you could dual wield Deagles and .44 revolvers without breaking your arms, and we aren't even going to get into damage done with dual UMPs or Raffickas. Seriously people sniper rifles need to be able to quickscope because it's their own brand of bad ass, every class has it.
That's just scratching the surface too, we could talk about One Man Army and infinite rocket launcher faggotry. Shut the fuck up about sniper rifles, game mechanics get abused no matter how 'realistic' they are.
