Well, on 360, i played it since release, and yes, as a sniper, i knew where to aim after spawn, as people always run the same way.
Grenades over buildings are easy too, as people always hide in certain corners.
You just learn it naturally, as you play more and more, you figure out how every weapon reacts, and your new to a fairly old game, so its no surprise your getting raped, as the casuals have left (That and your on PC) so its going to be fairly hard to get better when your playing with competition above your level.
Also, There are no Hacks for the Console Games, which is all i have experienced, and i have experienced what you are explaining.
They could also be Sound Whoring, where they can hear you round the corner, ever if your barely moving, reloading, scoping up, everything makes a noise, and some good headphones can let you hear this. Essential for Competitive play.
PC Gamers play with PC Gamers, Ps3 Gamers play with PS3 Gamers, 360 Gamers play with 360 Gamers.
You could also be experiencing lag, and how unaccurate the killcam is, its about half a second slow, so you can watch yourself die when you are round a corner etc. This phenomenon has be called 'Curving Bullets' because Lag makes it seem like they can shoot round corners.
