Geoff,
Some quick thoughts....likely not new to you...
Per your comment about rudder jerk...Erratic servo jerks have occurred when I initiate heavy amp draw. Resolved problems by grounding all circuits and components to a common ground bus.
Sound....I have tried all sorts of on-board sound and none give satisfactory volume when out on the water. Not tried yet, but an approach that may work is an on-shore, large sub-woofer. The sound track, with appropriate timing delay and echo effect, would be triggered as gun fire is triggered. Deep bass sounds from a subwoofer can be very non-directional, so perhaps the sound would seem to come from the model, especially with the visual cue of gun flash and smoke.