Sparrow (AIM-7) is radar guided by your aircraft, so keep the lock until it hits the bandit. AMRAAM (AIM-120) has it's own radar, wait for the ACT (ACTive) counter in the HUD to get to 0, then it's radar becomes active and starts tracking the bandit on it's own, after that you can break lock. Sidewinder (AIM-9) is IR guided, lock, fire, and forget.