My first thought was a space propulsion drive based on machine guns, much like ion propulsion. If humanity decided to get rid of all firearms (ha!), we could probably send them to space and drive a few spacecraft cheaply, easily and for a very long time (there are a lot more bullets than nuclear weapons)
Thermonuclear propulsion, as cool as it is, would be very powerful but uncontrolled acceleration, whereas a firearms-based drive could be controlled a lot more.
It would be fun if someone did a comparison on how long, fast and far nuclear versus bullet drives would get us in space.
Most bullet cases are made out of brass. I'm no rocket scientist, but brass cases would make a horrible rocket engine material.
The only reason cases don't catastrophically fail when you fire a gun is because of the bolt/chamber providing support.
Brass IS pretty good about stretching quite a bit before a catastrophic failure. This property is what allows case fire-forming (http://en.wikipedia.org/wiki/Fire_forming). It's also a pretty awesome safety feature.
Cases are so bad by themselves about containing the force of expanding gas, that ammunition fires aren't nearly as dangerous as you might expect them to be. (https://www.youtube.com/watch?v=3SlOXowwC4c) It's a long video, but if I recall correctly, it demonstrates that a rifle cartridge fired without a chamber doesn't have the energy to penetrate cardboard at like an 11 inch range.
I had a box of cartridges in a fire - they didn't go more than a few inches. The cartridge fragmented, like a firecracker with no net momentum to the bullet in any particular direction.
Thermonuclear propulsion, as cool as it is, would be very powerful but uncontrolled acceleration, whereas a firearms-based drive could be controlled a lot more.
It would be fun if someone did a comparison on how long, fast and far nuclear versus bullet drives would get us in space.