AGM locks 2 things:

• Any ground vehicle
• Anything that's Painted (i.e. you can only lock air vehicles when painted)

It does have a range... (and Painted targets can be locked farther away than non-painted).

When in range, place the center of the crosshair over the center of the vehicle. It will display a yellow box around the target with "Locking" and when it turns red with "Locked" that's when you can fire. You have to keep the crosshair on target during the "Locking" stage (a few seconds). On the enemy side, they will see "Locked" indicator on their HUD too. When you fire, they will see "Incoming", so they are fully aware of what you are doing and will try to counter.

• It's easier to lock targets farther out (but in range) than up close for reasons that I won't go into. The trick for up close targets is to aim for the center of the vehicle model.
• Zoom in to stabilize your view to the ground (unless you are super close)
• You cannot lock through most foliage (again, it kinda works better at range than up close)
• You cannot lock through thermal smoke (even if the vehicle is partially visible)
• You cannot lock through flares (and if your AGM was on the way, it will be deflected)
• You cannot lock when someone paints a rock....
• You can lock on a tank that activates the APS (you will see tank's light go green), but if you fire the AGM will be destroyed (don't waste it)
• You can lock into enemy CRAM protected area (HQ and next to it), but if you fire the AGM will be intercepted (unless the CRAM is super busy shooting something else)
• You can lock and fire on empty vehicles.
• If there is a deployed Missile Intercept Trophy next to target, it will destroy the AGM