Single, double, or safe?

No relation to baseball, promise!  Single, double, and safe are all types of actions on semiautomatic pistols.  The action refers to what happens when you pull the trigger.

 

Single action (SA) is the simplest.  As the name implies, when the trigger is pulled, one thing occurs.  That is releasing the hammer or striker pin.  This is the type of action see on most rifles and shotguns.  With a SA pistol, the hammer or pin must be manually cocked for each round.  This is more of an issue with revolvers, as most modern semi-automatic pistols have a cocking mechanism as part of loading a round into the chamber.

Double action (DA) triggers perform the cocking step during the first portion of the trigger pull.  Thus these guns will often have a longer trigger pull.  Most also have a noticeable change in the trigger pull weight from the cocking portion to the firing portion, with the firing portion being heavier.  In a double action only (DAO) pistol, there is not option to cock the gun manually and use the trigger only for firing.

Double action/Single action (DA/SA) is the most flexible trigger option.  This action allows for the use of the trigger in double action mode or for manual cocking of the hammer and use of the trigger only for firing.  On most semi-automatics with this action, even if the double action pull is used for the first shot, the mechanism of gun will cock the hammer for subsequent shots, making the trigger pull for rounds 2-n* single action.


Glock describes the action of their pistols as "safe action"  This means that the trigger pull incorperates the release of three safety mechanisms (1 external, 2 internal).  The first safety is that of the two part trigger.  The second is a firing pin safety and the third is a drop safety.  All of these are designed to prevent accidental discharges due to dropping or mishandling.  All are disengaged with a normal trigger pull and reengaged when the trigger is released.  In reality, Glock has a single action trigger with some fancy extras. 


Trigger actions and mechanism are rather difficult to show in pictures, and to understand the word descriptions of if you aren't familisr with them.  So, I'd recommend this video for those of you still confused or wanting more.

*n is a mathematical abbreviation for the highest number possible.  In this case I am using it since each firearm will have a different highest number of rounds, depending on magazine size and loading. Most of you reading this probably know this, but I figured better to clarify than confuse people.