"Destroy the AA guns (1 of 3)" is a mission objective in the game Aliens: Colonial Marines
There are a total of three AA Guns (Anti-Air Guns) you need to destroy
Yard with AA Guns
Zoom in on the AA Gun with your scope and fire three rockets on it. If you miss or if the AA Gun is not destroyed yet, go back to where you found the RPG and there will be more ammo for it there.