Don, that pistol port is a mis-nomer. It was extremely rare that someone would open it for any reason when enemy troops were in the immediate area. All that was needed was to fire a few rifle or mg rounds at it, and some would get in. It's extremely scary to have a single rifle or mg bullet enter and bounce aroud the turret a couple times. If enemy troops got on the tank, it was normally the tank in front or behind that would hose them off.

When you got the tank to a protected area, the port was opened and whatever you fired was kicked out. The rounds didn't kick out any distance..normally bouncing off the sponson box or wind up laying on the back deck. The idea of a mechanism to shove the rounds out is good. Best idea would to make a device to toss out a couple then close. You could also program the sound module to have a couple clanging noises when the shell casings hit the sponson boxes. If anyone could do it, it would be you.