One of the reasons why Call of Duty is very popular among hardcore gamers is because it allows the players to compete with other players through an open network. There are a lot of advantages of playing this game online and one is that you will be able to compete with more players without being limited by people who are near your place.
The first trick that you need to know is to remember the places of all your enemies. The players will usually start in a particular place, and remembering these places will help you kill the players faster. Use the Greenhorn Mode until you are familiar with these places and use this knowledge as an advantage.
Another thing that you need to remember is to try and use pistol guns as often as you can. When you are exchanging fire with an enemy and your primary weapon runs out of ammo. It is better to take out your pistol and use it instead of wasting time reloading your primary weapon. Pistols inflict less damage, but they are a lot faster compared to using primary weapons.
And finally, try to use as many gun combinations as possible. Try to experiment the best combinations that work for you. Always keep in mind that different players may need different combinations depending on how they play the game. The effectiveness of the weapon depends on how the player controls the shots.