Please allow me to rephrase a little of what I wrote previously.
I said it sounds like a typical rail gun. What I should have said was it sounds like an atypical use of what would have been the typical pattern rail gun, but instead of being a 1.75"-2.25" smoothbore, it was a smaller and rifled bore.
In light of what John has written, I'd say it is possible that it was a 1.25" Gatlin gun.