The real problem though is that "fire for effect" would be a barrage, not just one shot from each gun, so 2-3 minutes between shots is just as unrealistic as rapid fire...and much less fun
Of course, you could say that the F.O. only has one gun "assigned" to him, or that he is just calling in fire from one gun to conserve ammo. Or, you could have a 2-3 minute delay followed by a whole salvo of shots. Possibly it could be modeled by installing a bunch of invisible, weightless, etc. rockets.
It also occurs to me that a F.O. calling in fire from a ship might also have fewer guns to fire for effect than if he was sighting for an artillery battery. Depending on the ship's position and bearing relative to the target, only 1-3 guns might be able to bear, especially if it was a destroyer operating close to shore.
Anyhow, enough "hand waving" about how to justify the effect. This is an amazingly fun mod, and proof of why an arty spotter plane was the most dangerous ground attack aircraft in the sky!