This mod has nothing to do with actual artillery. All I've done is mounted some invisible, no recoil, no tracer, no weight cannons to the a/c.
It's a cool idea, but I can hear the on-liners screaming already. (Yeah, I know SAS is mostly for off-liners, but . . .)
To make it a bit more believable and to tone it down a bit, would it be possible to put a delay fuse on your shots (like those for rockets or bombs), so that they only go off after a 2-3 minute delay?* That would sort of simulate your FO calling in the target to the battery, trajectory calculation, battery response time, shell flight time, ranging shots, etc.
* During WW2, American artillery was the most advanced in the world, and by the end of the war U.S. artillerymen were some of the best, but forward observers still had to estimate range to the target, call in grid coordinates and correct aiming point by observing the fall of ranging shots. On the other end, battery officers had to calculate trajectories using mechanical computers and "dumb" munitions. Then the gun captain had to choose the right shell type, set the fuse by hand, and, in some cases, estimate the correct amount of propellant to use for the shot. This all meant a 2-3 minute delay - at best - between the F.O. calling in fire and "fire for effect" by the whole battery. With less efficient communication, bad maps, less experienced gunners, and more primitive equipment, it could take twice as long - or longer if the battery had to move, was under fire itself, or if other F.O.s had priority.