Apparently, if the novel is to be followed, it's a some sort of a giant magazine-fed Gatling cannon, which the chaeronian Castigator has reconfigured in order to make it capable of firing demonic shells. I think loyal Castigator(s) will either leave the gun as-is or reconfigure it in order to channel the faith of the crew ans charge the shells with it before firing, especially after what their "brother" has done.