Do you have a dremel?
I made a jig with a door hinge and some scrap wood.
Here is a pic of the first one I made....its rough but worked great.
Short video of it in action cutting 223 down to 300 blackout length.
http://www.youtube.com/watch?v=yPU9j...cdE7qg&index=1
Here is the one I built after thinking about it and buying a 2x4. It has a 32" routered groove down the center to keep arrow shafts straight....after a quarter turn the arrows are perfectly square. I haven't used it on 223 cases yet because I cut over 1000 with my first jig.
I piece of plexi glass wouldn't be a bad idea for brass to help keep you from getting hit by chips. Dremel has a heavy duty reinforced cut off wheel seen above that will last about 500 cases.