Rigidchips is a program that falls under the category of "rigid body simulator". Using simple text files or a third party 3D editor, you can make 3D objects using 2D squares that have different properties.
And yes, you can put guns on your creations and fight to the death.
Here is a download link for the game:
http://www.iamas.ac.jp/~takeya04/softwareE.html
Here's a download link for the 3D designer (Winrar or similar zip program required):
http://hp.vector.co.jp/authors/VA013928/bin/rcd022.lzh
The English-speaking forums are found here:
http://rigidchips.forum-motion.com/
If you don't trust the links I've provided, you can find this information on the Wikipedia page for Rigidchips.
If anyone is interested in playing some multiplayer, just let me know and I'll schedule a time that I'll be hosting.
Do a google image search for "Rigidchips" to see more pictures of awesome creations people have made. I would post some here in a spoiler, but these forums don't like the extensions that most of the pictures use.
Some videos showing off what Rigidchips is capable of: