Algodoo

The entire top part angles and slides out and when it does that you can put the bullet into the rear of the barrel (No collision but still opaque, i'll fix this later)
Make sure the bullet stays in place, otherwise the killer wont hit the bullet and nothing is gonna fire
If you've loaded the bullet sucessfully, pull the trigger to fire (duh, what else is gonna fire it?)

PS: and the bullet may misfire aswell but again, i'll fix this later because of the limit of how much saves you can put into the Algobox/day

very clever design