Before I moved on to pre-compessing the spring as shown, one of the all-thread rods snapped causing the other to immediatly snap, too. At the time I had the spring on the floor with my foot on it, so all the drama happened to the left and right of me rather than through me. The projectiles could easily have gone through me had any body parts been in the path. The visualization reminds me of a joke:
Q: Why did they stop the leper hockey game?
A: There was a face off in the corner.
Makes you wanna run out and get one, eh?
Comparing this design to the pictures Jim posted, you can see the advantages of supporting the all-thread rod with a sleeve.
Somewhere else Jim pointed out that you have to be careful not to compress the spring too much, lest you damage the spring itself.
-Evan