Well, this thread is kinda old, and I'm sure X2 is getting all the focus, but this is the place to post this, so...
(And for all I know this entry carries over to X2, so it might still be pertinent)
In the Xenopedia entry for Rapid Fire Lasers, we can read this, amongst other things:
"The proposed solution borrows heavily from the Autocannon mounted on our interceptors - multiple rotating barrels. Instead of using a rotating barrel arrangement to facilitate high-speed loading of ammunition, it can be used to vastly reduce the amount of heat continual fire will generate in each individual barrel."
This would seem to imply that the purpose for multiple rotating barrels in the autocannon is to facilitate high speed loading of ammunition. But this has never been the reason for multiple rotating barrels, in any weapon. The reason gatling guns have multiple barrels is precisely to reduce the amount of heat generated for each individual barrel - it's a cooling mechanism. There is nothing preventing the design of a single-barrelled gun with the same rate of fire as a mini-gun, but without the rotating barrels you'd overheat 3-6 times as fast.