Get on a motorcycle. Then drive to the river.
Now follow the river until you get to some rocks and barbed post. Next drive into the right hand corner so the head light is right in the corner. Move over to the passenger seat and get out. You should end up on the other side.
Now sprint between the two rocks. Now sprint and jump on to the big rock touching the section of grass. When you're on the grass crouch (B button) and push on the wall.
Then start moving froward into the bushes but keep pressing against the left hand side. Sometimes you might get stuck so just wriggle your left thumbstick about. Eventually you'll come to the end and you will end up slipping off behind some barbed wire stuff.
Run onto the bridge and look at where the barn would be. You can shoot straight through the hole in the hill!
Note:If you go further than the end of the bridge you fall through the map. Also when you're trying to jump onto the rock, sometimes you might get stuck and you have to kill yourself or respawn.