Though there are lots of different types of trigger locks, perhaps the most common variety is a small plastic or metal device that fits over and through a gun’s trigger guard, impeding one’s ability to shoot. Like a padlock, an external trigger lock can only be removed with a key or the proper combination. In 2008’s >District of Columbia v. Heller, the Supreme Court ruled that a D.C. law requiring gun owners to use trigger locks violated the Second Amendment, so we probably won’t be seeing mandatory trigger locks anytime soon. But, even so, the devices are inexpensive and easy to produce and distribute, and if governments or gun groups wanted to do so, they could easily buy trigger locks in bulk and distribute them for free, like condoms.
Source : http://www.slate.com/blogs/crime/2013/07/18/trigger_locks_the_dubiously_effective_safety_measure_that_gun_control_advocates.html