There are a variety of reasons that you will rage in these 2 games. One would be the very infamous noobtubes. They can piss people off so much. It is very easy to get angry because they will shoot the launcher making it explode near you. You can die quickly from these and with One Many Army, they will have unlimited ammo.
Another reason would be claymores. You may accidentally step into one and explode. Or you have seen the claymore, but accidentally stepped in it anyways. People may also place claymores in the back to protect them, but the claymore fails and enemy gets behind you. Claymores are very unreliable.
12 year-old kids playing the game will get you really angry. They always talk back and their shrill little voices may get to annoy you a lot. You may want to argue with them and start yelling at them for being retarded. They will think they are really good at the game and start to yell and scream that they are better. Overall, these kids may give you a huge headache.
My last reason for raging would be the air support. You may be in a streak and all of a sudden, a "harrier" kills you. You will start yelling that you "almost got a nuke". You may have just gotten an air support, but it does not kill anyone and you have to continue on your killstreak without the help of your air support.
One of the most popular is the Famas. It does a lot of damage(4 shots to kill) and surprisingly does not have that much recoil. It is used by so many people, you will see it kill you in almost every single game. It is a automatic instead of a 3 round burst (like mw2).
Another gun that is probably used a lot, but not as much as the Famas, is the Aug. It was known as the Aug Bhar in mw2 and was a light machine gun. It shoots very rapidly and with grip, barely has any recoil. This is an ideal weapon in most situations.
