Call of Duty 4: Modern Warfare

Don't use RPGs. They curve off course too much.

Point your gun at it and pull the trigger. If you have an assault rifle or an LMG, that's preferable to a sniper or a shotgun. A couple of people shooting at a helicopter with ARs will down it quickly.

If you have RPGs try em but i wouldn't suggest them
and i geuss just unload on it and hope that ppl on your
team are doing the team so you can bring it down faster

Use LMGs to take down the helicopter. The long clips full of 100 bullets will take it down in no time. If you get a full clip on the heli it is dead in hardcore or hurt badly in regular. Also if you're on a map with mounted machine guns, they work the best on helis. Backlot the gun could reach it but only if it's low and in front of you. Downpour and Ambush are the best to use the guns on because there is one on each side to shoot it down with.

honestly no matter how hard it is, RPG's is the fastest way. the helicopter stays in place every few moments to stop and shoot. when it does take a little cover (possible kneel) pull out the RPG and when aiming try not to move the launcher so much. pause so your completely still and fire it at the helicopter and dont move until you have either hit it or the rocket veers off course. the closer the better. after you hit the rocket once fire it it with you main weapon a couple times to knock it down. thats if you even have to.

lord gamer is right, no matter how hard the rocket is to aim, its the best way. it stays in place every few second when it does, dont aim at its center aim towards the side or bottom or top. the rocket moves in mid air so thats why. once u shot it (if u have an M9 pistol) pull out your pistol and start shooting it.

hope this helps =D

Use a rpg. One shot from it and 3 pistol shots takes down a helicopter.

It all depends on the map. On maps like Bog and Countdown, where the Heli is in open view of the players at nearly all times, finding some cover to hide behind and get an RPG off is quite easy.
On other maps such as Chinatown or Crash (more sectioned off), you're going to want to open fire with an LMG from good cover.
On Downpour, there's no reason not to use the heavy machine gun on the chopper, though it's much less dependable on maps like Backlot and Overgrown where the Heli only comes into the HMG's range every once in a while, and on Chinatown it's absolutely useless.
DO NOT just spawn, get out in the open for a line of sight, and hold the trigger down. That's a really good way to get peppered by the heli's turret.

Remember that your position on the scoreboard, and your weapon to a much lesser extent effects the Heli's shooting preferences, so don't expect a walk in the park if you're MVP.
I hope this helped.

Heres what you do. if you have alot of players on your team the Chopper will probably be busy with others on your team. so get an RPG and head to the back of it then if you can shoot the belly or tail of the chopper. try not to use propeller shots though.