Striker fired guns can be single action (XD), DAO (Glock), or DA/SA (Walther P99 AS).
NOT TRUE.. I had been searching for answers after looking at several autos (GLOCK 22, Springfield XD, Smith MP) claiming DA/DAO, but noticing striker/firing pin only released after racking slide, inadvertently cocking the striker/hammer, unlike a true DA/DAO, such as revolver.
I have found one place to accurately describe striker fire action from NRA Certified Pistol Instructor.
Why Glocks are NOT Double Action
http://members.cox.net/guntraining/glocks.htmbelow are a few definitions and highlights...
Glock claims their guns are "Double Action Only". They are in fact striker-fire, SINGLE action, no matter what Glock and the box says. This misunderstanding has annoying operational implications, and possibly dangerous defensive implications.
Definitions:
Single Action: Pulling the trigger does ONE thing : if the hammer has been cocked, it will release the hammer. Trigger pull is light, short, and consistant between first and subsequent shots. Example: Star Firestar Plus.
Double Action: Pulling the trigger does TWO things : it will first cock the hammer if it hasn't been cocked, and then it will release the hammer. DA Trigger pull is longer and heavier than SA, but the user can cock the hammer first if they want to fire in "Single Action Mode." If the gun is semi-automatic, the first shot is DA, subsequent shots are SA due to the slide cocking the hammer for you, resulting in differing trigger feel as described above. Example : Beretta 8045, Hoekler and Koch USP series, etc.
Double Action ONLY: Any DA gun which does not allow pre-cocking the hammer because the hammer won't stay cocked, and is often bobbed or hidden. No decock is ever needed, and trigger pull is consistent between first and subsequent shots. Example: AMT Backup
Striker Fire: Rather than a hammer, an internal striker is cocked and released to fire. The striker is cocked by the first motion of the slide, and there is no way to decock other than squeezing the trigger.
Any "true" double action gun will cock the hammer by squeezing the trigger, which means you don't HAVE to cock the hammer manually before the first shot as with a single action. In contrast, a Single Action gun you have to cock the hammer before you squeeze the trigger.
Glock claims to be "double action only" because :
1) You can't cock the hammer from a "hammer spur" externally, normally a sign that the gun is DAO.
2) In normal practice, pulling the trigger makes it go off, suggesting (falsely) that the trigger is cocking the hammer like a DA.
3) Trigger pull feels consistant between shots, suggesting DAO.
All three are wrong because :
1) Glock can be (and is) cocked externally for the first shot, but its done by the slide motion involved with loading the first round into the chamber rather than an overt hammer. This is true of any other semi-auto gun -- unless it really IS a DAO gun, any semi-auto can be cocked by racking the slide if you wanted).
2) The gun IS in fact cocked for the first shot before the trigger is pulled, and stays that way once cocked until fired, just like any other SINGLE ACTION. The second shot is cocked by the first, etc.
3) Just as with any other NON-DAO semi-auto, Glock counts on the process of chambering a round to cock the gun for you between shots. Since you always fire in single action mode, the trigger always feels the same.
How can I prove my Glock is Single Action?
There are a lot of Glock afficianodos who will SWEAR that their Glock is DA, until I have them dry fire twice in a row to similate a misfire. Remember what I said the definition of a DA gun is? DA cocks the hammer for you from the trigger, and will do so every time.
I said before that Glock's claim to be DAO has gotten people killed.