I believe the mission name is Faralon but maybe not.
It is the mission on the Dam where 2 havoc helicopters attack near the end of the mission.

I can not seem to shoot down one of the helicopters.
I get the first one with my missile, I even have find a RPG but it does not ever hit the helicopter. I shoot at it and hit it with 1000 rounds, because I use every dropped gun I can find, but the 2nd helicopter just smokes and never goes down.

I have played this mission 50 times and nothing seems to shoot down the 2nd helicopter.

Any ideas?
Thanks