Algodoo

This one's good!
If you want some inspiration, check out my guns. You should cut the box where the bullet should be loaded so that it fits perfectly

But... how are you doing this *-*

how this works
if you look close at the bullets you will find a spring
that is what makes the bullet work and there is a part holding the bullet together
the trigger kills that part making the spring expand and another part kills a part on the bullet head killing the spring
and the trigger i scripted
set the sim speed to 0.1 and look close at the parts while fireing the gun
if you want more info you can ask one of the good gun builders for help