Well here is a brain storm idea that you can run with if you wish.

Picture one of those cheap dollar store toy guns that shoots the dart with the suction cup. Only in this case we use a small gear head motor that turns two cams. As the first cam rotates it opens the hatch. the second cam draws back a plunger iside a bit of brass tubing that has attached a spring (light spring) as the plunger is drawn back, the "chamber is opened an a single spent casing is dropped in. As the cam continues to rotate the sring passes over a release. The spring tension poops the casing out and as the first cam completes it rotation the hatch comes shut. A micro-switch just like the set used on the recoils then trips and resets the effect.

YOu will have to add at least a 320 Ohm 1/4 watt resistor to get the speed just right.The hatch cam will haveto be largerthan the cam that ejects the shell casing, and you will need to design a shell feed system.

This would be a great little project!

Don