I found some inexpensive (~$5) automotive door lock actuators on Ebay, and had to try them out for various robotic/automation needs. I'm always needing some way to get my PIC uControllers to do actual real-world work. Here I simply have an IC Relay chip controlling the 12V to the doorlock piston. The IC is PVN012 ($3.25 at http://www.digikey.com/).
It worked great - in fact, using only 5V the actuator would move almost the full 0.75inches.
So... because it is SPRING, and time for Airsoft battles, I had to hook one up to an airsoft gun. Its usually me against the rabid mob of kids, so I need something to help 'even the odds'.
You can see it easily mounted to the grip of my MP-5 automatic, and even is out-of-the-way enough to still use the weapon by hand.
Now... I can quickly put this at one end of the house, and chase kids around the other way right into the sights - then with the push of a button, wirelessly catch them in the cross-fire!
Here's the video - its even better. Best of all, it is all less than $10.