Chopper will most likely be the last weapon you unlock in the game, as its requirement (get 1,000,000 XP) cannot be fulfilled until you are well past level 100.
The weapon works similar to a Seagull that can aim its missiles. When fired up into the air, the Chopper releases a missile approximately once every .25 seconds, which automatically locks onto the nearest enemy target, dealing 4 damage when it hits. The Chopper itself deals 20 damage if it connects-- its radius being in-between the size of a Tunneler and a MegaTunneler.
Apache is the level 2 upgrade of chopper. The Apache's fires at the same rate as Chopper, however its missiles do 5 damage per hit, and the Apache itself does 25 damage if it connects-- its radius being the size of a MegaTunneler.