Weapon lock is a system that alerts the user of a guided projectile weapon that automatic tracking of a target vehicle has been acquired, and that the fired round will follow the target to the best of its ability.
In the modern- and future-era Battlefield games, all vehicles except light ground transports and non-combat vehicles (such as the Skid Loader) have some sort of warning system that detects the use of guided weaponry. The system uses a visual indicator and an audible klaxon.
Vehicles can detect the rangefinders used by Engineers for their default AV launcher and the SAAW 86 Anti-Air, and by enemy vehicles for their main weapons (tank main cannon, railguns, or rocket pods).
The lock alarm does not actually warn against the projectiles used, but rather that an enemy is aiming an AV weapon towards it. There is also no indication of where the offending launcher is located. Anti-vehicle infantry can thus spoof vehicle operators by merely aiming at them (even with empty AV weapons), causing them to waste countermeasures or even leave the area to avoid conflict. Skillful users can also aim off-axis until their munitions are close to the target, or fire unguided to prematurely end the alarm.
Players can make use of heat-seeking weaponry such as AA missiles, the FGM-148 Javelin, and vehicle-mounted guided missiles. They can also use laser designators to mark targets for laser-guided weaponry—the weapon carrier must acquire a separate lock while the designator himself is locked onto the target vehicle. A rising chirp signifies active seeking, with a steady high tone indicating lock.
Vehicles can detect both locking systems, identifying it as a low beeping tone that goes solid when fully locked. They can also detect incoming missiles, identified as a level high beeping tone. This allows the operator to use countermeasures such as IR Smoke or flares, or to break the lock using ECM Jammer. A skilled operator can also make use of overhead cover against top-attacking laser ordnance, or lateral cover against heat-seeking weapons.
Wire-guided weapons such as the BGM-71 TOW, 9M133 Kornet, or an attack helicopter's TV missile do not trigger an alarm, as they do not rely on weapon lock. However, wire-guided weapons do not travel as fast as other guided weapons.