Being an anti-missile is more-or-less a player description. You can use your AMMs to attack other ships (sometimes very powerfully, as you just sandblast their armor) and you could use your ASM to shoot down incoming missiles, though very poorly since they tend to lack the agility to hit incoming warheads. Just make a small missile that's fast and agile, the in-game description will change to "AMM" automatically once you pass some threshold of size, but this is purely descriptive. The actual function is what the player decides.
