Im playing call of duty 4, veteran, and im on the level where the stinger crashes your heli with price.?

now, i've almost beaten the level, but theres this damn ball busting part i cant get by!! UGH! its the part where after you sneak past the senteries, and when your about to go out to the barn to get the stinger and shoot the heli down. but i CANT GET THERE! i can get as far as the broken down building to the left but the heli just instantly aims for me and kills me. is there any tips to get by it? also, the guys inside the barn killed me when i got in. help? really appreciate it