Well campers have 2 disadvantages.
-No element of surprise. If you run into a room carefully, you can see a camper faster than he can see you. Considering average human reflexes, you have the advantage of killing camper before he/she kills you.
-Lack of Offensive. Like i always said, Offense is the best Defense no matter how you look at it. If you sit back and camp, your team is going nowhere and all you are going is racking up death ratios. The enemy will eventually flush campers with grenades.
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For new players, camping is usually a common tactic applied because of disadvantage of a unfamilar map. So by camping they can see where the enemy is coming from and sooner or later they'll learn to attack.
For Rifleman, snipers and Machine Gunners. Rifleman is bolt-action and semi-automatic. Look at from that angle, would you go face to face with a group of Automatics? The best you can do is take down two and the rest make you into Cheese.
Rifleman do not go head on and require a bit of " defensive camping." Rifleman go in offensive as a support with the automatics on the same team. They are hard to assault alone. Rifleman in my opinion is the defensive class, going into a small room with the enemy strafing you isn't the best situation you want to find yourself in. Conclusion, as long as the rifleman displace every once in a while to get a better shot, i do not call that camping.
As for snipers and Machine gunners, well machine gunners you can't expect him to go running around ramboing. His role is to set up his machine gun and mow enemies down. Snipers are hard to assault alone, since no scope does not give the sniper a crosshair and pistol is useless at medium-long range. Camping is merely a defensive tactic.
So think about it....Don't always hate on campers.