Telescoping Tubes

Inspired by extending tubes (http://www.thingiverse.com/thing:7081).
I wanted to use less material, make each tube thinner so more will fit in the same area and have the tubes lock in place when open or closed.

When the tubes are closed the guides that protrude into the track of the tube around it are held in place by a bump in the track (like in thing #1481).
When the tubes are open they twist to lock and the guides go over another bump in the track so it can't fall back. Having it twist to lock allows the track wall so it can resist force along the axis.