Hey Terry,
I think I have some more information for you, at least on the firing of British heavy guns. This is from the video I posted of the guy closing the breech on a WW I-era 15" naval gun... I've done a bit more research/reading, and I THINK I have the following information correct...
I've circled two areas in red.
The circle over the center of the breech shows the primer area. In this case, the full assembly is there and is closed as if the primer is in the mechanism and is ready to shoot.
You note the black cable going to the brass clip on the breech block . As the breech rotates counter clockwise, it meets up with another brass clip on the standing breech (the other red circle), which also has a cable going to it.
I'm about 95% sure that that is the electric firing interlock. When the breech is fully closed that interlock sends a signal back to the fire control center that the gun is ready to fire. When the gunner in the fire control center presses the firing trigger, an electrical current passes back down through the same cables to the primer cartridge and the gun fires.
If the breech block isn't fully closed the firing interlock circuit isn't complete and the gun can't fire.
After losing several battle cruisers during the battle of Jutland, in large part due to grave errors in the handling of the powder bags, including leaving the flash proof doors open, the British REALLY ramped up safety on their post war designs leading into WW II and created a whole series of electrical interlocks that would, were any of them not showing ready, prevent the gun from firing (others would prevent the power hoist from operating, etc.)
The system was complex, but it helped ensure that an accident in the turret couldn't flash down to the powder magazines.
The complexity of the interlocks (as well as other issues) were still being ironed out when the battleship Prince of Wales sortied out to meet the Bismarck and those issues cut PoW's effective rate of fire. Because the issues were ongoing the PoW's captain requested civilian technicians from the Elswick Ordnance Company, which developed and built both the gun and the mounts, to remain on board. I've read varying numbers, but apparently upwards 100 technicians were on board during the battle and actively participated in operations, helping keep PoWs guns firing until one of the quad-mount turrets jammed.