Algodoo

Version 2 has a number of changes.

First, taking Ravenplucker's suggestion, I spring-loaded the feed mechanism. The gun will work in zero gravity and sideways (sort of, still working on upside-down).
Second, I added a cartridge system so that there's no door holding the bullet in. Again, this allows the gun to fire from more angles.
Third, I shortened the gun by a lot and reduced the unnecessarily large densities. The gun is now 93% lighter, and moves noticeably when fired.

I can't think of much more to add to this gun, maybe a system for swappable clips? If I think of anything good, I'll be sure to post it.