[lutusp]

In the technical literature, the term "rail gun/railgun" is strongly associated with electromagnetic propulsion methods:

http://en.wikipedia.org/wiki/Railgun

[mseebach2]

Ah, I was thinking of a Gatling gun (http://en.wikipedia.org/wiki/Rotary_cannon), not a rail gun. The concept of rotary tubes seemed to mesh with OP's vision.