In the anime Outlaw Star, a Caster is a powerful and very rare weapon. Hundreds of years ago, magicians were a common sight, but as time went on the mana which powered their magic slowly depleted. To prevent their power from being lost completely, they created Casters. A Caster fires shells, which, after so long, are quite rare and expensive. There are twenty different kinds of shells, each with a different effect. Some, such as the #4 and #9 shells, are so rare that it is thought that none exist any longer. Some are fairly "boring", just being big explosions. Others are quite interesting; the #4 creates a black hole, which sucks it's target into nothingness, and then vanishes.

The design of a Caster gun varies. Gene's is shaped like a large, long-barrelled pistol. Another one seen in the series is much stranger, about 3 feet long and looking rather like a wand of some sort. Sorry I can't describe these better, I'm always horrible at describing mechanical devices clearly.

In the current time in the series, the secrets of the Casters have been long lost. In fact, Gene Starwind did not even know that the Casters were magic until he met the creators of the Caster, three extremely long-lived magicians living on the planet of Tenrei1. They revealed to him the secrets of the Caster, and in return for some saucy video of a woman they had the hots for, they created for him several powerful caster shells. The mana was so depleted by this time that the shells would take much of the needed energy directly from the life force of the one who fired it; Gene nearly dies while using the Caster to fight the Anten Seven in the Galactic Leyline.

The shots of two Casters will cancel each other out, unless one of the shells is much more powerful than the other, as Gene found out when he went up against Ron McDougall, a fellow Caster user. In their first encounter, their shots simply vanished when they hit each other. After Ron pointed out that he had many more shells than Gene did, Gene decided on the very sensible course of action of running away. The second time, however, was shortly after the visit to Tenrei, and Ron died a nasty painful death (though they brought him back in the end, in what was seemingly a setup for a second season).

While the Caster is exceedingly powerful, it can be defeated; a master of Tao magic can dispel the attack. In this case, not only does the shooter waste time (during which the magician can mount their own attack), but a valuable and rare shell is gone forever. Fortunately, Tao magicians are quite uncommon, and generally only found among the higher ranks of the various pirate groups. A non-magician, Aisha Clan Clan, survived a Caster shot to the head, but given the general toughness of the Ctarl Ctarl, it's not really that big of surprise. Ron

1: The episode explaining this important and otherwise not obvious point, Hot Springs Planet Tenrei, was not shown on Cartoon Network. Aside from exploring the history and powers of the Caster (and Gene picking up some powerful Caster shells), the episode was complete fan service, including showing Aisha naked and lots of other girls in various states of undress. You'll have to buy or rent the DVD to see it. :)