Algodoo

Don't try to find any sort of reason behind this mechanism. It works, and it's more reliable than most of my other guns, even though it runs at 300 Hz and is way too overengineered. The basic principle behind it is to eject the empty casings with a spring-loaded hammer thingy (the center lever acts both as a bolt catch and as the loaded hammer) right after the bolt is tilted 60º. After that the bolt gets caught until you act the center lever or until you push a round into the cylinder-chamber thingy.

There's already one round on the chamber. To fire, simply disengage the safe (put the first lever in F), then pull the trigger or press W. After the round gets ejected simply put another one on the cylinder chamber and either push it down until the lock disengages or pull down from the middle lever.

There are some boxes for you to shoot at (sort of).