Algodoo

8/10 for looks.
6/10 for functionality (magazine wouldn't stay in, trigger is a little strange).

Overall, 7/10.

I suggest using a hammer and firing pin to kill your red box in the rounds, instead of a slow rotating killer on a motor. It'll be much faster, more reliable, and more realistic.

And I'm not sure if there's a mechanism to keep the magazines in or not, but I'd add one or improve the one that's in there, because with the scene running at 60Hz, the magazine wouldn't stay in at all.

Use arrows to keep the magazines.;)