Appears to show compressed air [scuba tanks] under the gun to simulate firing during filming. As well as what appears to be wires for the solenoid valve.
Indeed! The gun doesn't seem to be mounted on its trunnions (which appear to be exposed behind the axle). I wasn't aware of any mounting scheme that strapped the barrel directly to the carriage axle, either...
I'm no muzzleloading artillery expert, but that appears to be quite a movie kludge"...