Well, basically, to take the firepower of the entire fleet and put it on one target. You could throw in extended range (have the MBC's reactor basically put all its power to create an extended beam that serves only as a conduit for the other beams' power to the target). Or something. It's really just and idea; you'd have to flesh it out. I think everyone agrees it would be kind of cool.
It should have higher armor piercing value, too, as all the firepower is concentrated on one point, and, as good as any armor might be, repeated or intense damage to one point will usually overwhelm that point.
Here's an idea: load up your favorite capship slugfest mission. Now count the different anti-capitol beams and types available (roughly). Then figure out the DPS if they were all firing at one target at one time.
Actually, another cool idea... heh.. have some sort of assembly of capships around a docking ring. When in the ring, they fire their beams at the central part of the ring (ring is not hollow, there is a mechanism in the ring) and the ring then fires the combined energy. After several of these shots, as the ring is closing in on the target or bombers / fighters are getting close, the capships undock and scatter, creating separate individual fields of AAA/flak fire, and reducing the chance of being all destroyed one after the other.
Anyways, these are all just random ideas. If you like 'em, great. If you want to use them, swell. If you don't... erm... then you are in the wrong thread? Unless you just want to discuss potential lack of benefits or disadvantages. However, at this point, being as it's just a concept, that's hard to do because no one knows how this would actually be implemented tactically and what kind of techno babble would be invented to justify it.
