I went to the NRA convention last year and saw a gun lock that utilized a dummy cartridge kept in the chamber. Before using the gun, all you have to do is rack the slide (round is ejected) or advance the cylinder one chamber. If the trigger is pulled on the dummy a spring loaded post/bullet sprang forward and locked up the gun. To unlock the gun all you have to do is use the tool, or any rod smaller than the barrel, and push the post/bullet back into the cartridge and rack the slide ... Sales pitch is quicker into action and/or lets you know someone was fooling with your gun, providing they don't know the trick.
Anyone know the name of this device/where to get them?