Ah, I was thinking of a Gatling gun (http://en.wikipedia.org/wiki/Rotary_cannon http://en.wikipedia.org/wiki/Rotary_cannon), not a rail gun. The concept of rota

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