From what I've read, the reason the XM-8 was developed form the OICW was that they were having a lot of problems with the electronics in the 20mm cannon of the OICW. Don't believe everything you see on the history channel or read in Popular Science... the military develops a lot of things all the way to prototype stage or even a small order fielding, without them becoming the standard.
Last I read, the OICW was still in the works, but postponed for a while... and the XM-8 is looking like the model for the next replacement. There is still some debate over the caliber, however, since a lot of former grunts are calling fro something larger, like 7.62mm. The same article I referred to above mentioned they had built some prototype XM-8s firing 7.62.
Slayer, the gun you are probably refering to is .... damn, the name's escaping me, but you would need it for point defense against incoming missles. We already discussed this a while ago, but it would be used similarly to the CIWS ("Phalanx") system now used onboard ships, which is a high rate-of-fire gattling gun used to basically disinitgrate incoming missles.
Photo of Phalanx Gun
PS Watching the videos... this is a great gun! Gotta love the ease of assembly, the storage spot for the pins, completely ambidextrous operation, the mags that are translucent and can lock together side-by-side, fires dirty and wet, and fireable one-handed full-auto! Kick Ass! Too bad it'll probably be 2030 before the Coast Guard ever sees these.