Trigger Mechanisms; A Guide to What's What

In the past quarter century semi-automatic handgun design and development has exploded. There are so many trigger mechanisms out there that it's confusing to even experienced handgunners. In this thread I'll try to cover the types, names, basic operation, and major examples of each.

The Basics
We'll start with the three primary trigger mechanisms and build from there.

SA - Single Action. In this mechanism the firearm's hammer is fully cocked, and the trigger pull performs the single action of dropping the hammer to hit the firing pin and fire the round. These pistols are generally carried "cocked & locked", where the hammer is back and safety is engaged. Well known examples of this are the M1911, Browning/FN Hi-Power, some variants of the HK USP, SIG P-Series SAO pistols, CZ-75 SA, and FN FNP SA.

DA - Double Action. In this mechanism the trigger pull performs the two actions of cocking the hammer, and then releasing it to fire the round. The British often refer to DA as "Trigger Cocking" for this reason, and it helps many folks to think of DA in this way. DA autos are further split into three other categories as follows.
DA/SA - Double Action / Single Action. These pistols fire the first round Double Action, and all following shots are fired single action. They generally use a decocking lever to safely lower the hammer, but block contact with the firing pin so the chambered round is not fired.
Caution Decockers can fail, so always point the pistol in a safe direction when decocking. Decocker failures are rare in modern firearms, but do occasionally happen. Some older designs are more prone to failure. If you have any doubt that a decocker is not working properly have the firearm checked by a competent gunsmith or armorer.
DAO - Double Action Only. In these pistols the hammer always stays down, and the trigger operates only in Double Action.
Many well known brands of pistols are available in the Double Actions. Some are only available DA/SA or DAO, and many are available in either mechanism. Well known examples include SIG, H&K, CZs and Clones, Ruger, Taurus, Kel-Tec, Walther, FN, Beretta, and Smith & Wesson metal frame autos.
DAO with reduced trigger pull - These are the new breed of DA pistols, pioneered by Para Ordnace's "LDA" - Light Double Action. These pistols keep the hammer down, but cycling the slide pre-tentions the main spring / hammer spring. The result is a lighter weight DAO trigger pull, and some also provide for a shorter trigger pull & reset. Some also have the ability to revert back to a full weight DA trigger pull to give a second try if the round does not initially fire - this is known as restrike capability. Beyond Para's LDA, examples are SIG's DAK, H&K's LEM, and some Kel-Tec and Smith & Wesson Pistols. Edit: It has been brought to my attention that the hammer rests in a partially cocked position on the Kel-Tec PF-9, P-32, and P3AT; and possibly a few other pistols as well

SFA - Striker Fired Action. These pistols use a Striker, basically a hammer & firing pin in one. Most operate similarly to a lightened Double Action. Cycling the slide partially pre-cocks the striker, and the trigger pull finishes cocking the striker before releasing it to fire the round. Some pistols have the striker pre cocked only a small to moderate amount and are much like DAO in a traditional hammer & firing pin pistol. Others almost fully pre-cock the striker and are much like SA in a hammer & firing pin pistol. Others yet have striker mechanisms that are much like DA/SA. Glocks are undoubtedly the best known SFA pistols; others include Springfield Armory XD, Walther P-99 / S&W99, S&W M&P and Sigma, Taurus Millenium, Mil. Pro, 24/7, and CZ 100.

No doubt I've left some things out. If I've left any glaring omissions, or made any major or minor errors, please send me a PM and I'll edit to correct them.

If you'd like to add anything please feel free to post a follow up. If you could, please also stick with the definitions of trigger/operating mechanisms I listed above to keep things simple and avoid confusion. In your follow ups please stick to objective information and avoid being subjective. We can debate our preferences & the merits of the various mechanisms in another thread, but please keep this focused as an informative reference.

Thank You.

Edit 17 Jan 07: I corrected the first glaring omission I noticed and added Beretta to the major examples of DA pistols along with a couple of other tweaks

 

More Detail on SIG's DAK

THR member Fastbolt recently made a great post in another thread about SIG's DAK. He tells me that the info is basically what's in the SIG armorer's manual with a little of his own commentary added in. The post also offers his thoughts on traditional DA/SA vs. the new modified DA systems from an experienced perspective. Good info; http://www.thehighroad.org/showthread.php?t=242758

 

The QA - Quick Action - is a light DA version of SFA, much like the system Glock uses. The AS - Anti-Stress - is a SFA that replicates the feel of a hammer & firing pin DA/SA with a long pull first shot, and short pull on subsequent shots.

The "New York" trigger on Glocks is a modification that increases the trigger pull weight. The various weights of Glock trigger are provided by connectors or varying geometry and/or connector springs of varying weights. As far as I know trigger pull length and reset remain the same. The folks over at www.glocktalk.com can give you more detailed information on the many Glock OEM and aftermarket trigger modifications that are currently available.

Please let me know if I can be of further assistance. If anyone has more detailed or accurate information please feel free to post it.

 

Still a DA because the trigger pull both cocks (the rest of the way) the hammer and releases it.

Good call and CJ did a great job of explaining the Taurus SA/DA so I'll leave that as is.

You folks check my facts, but I believe the DA/SA Walthers, DAK SIGs, and LEM HKs also revert to full DA to allow second strike capability.

Of note there is some debate as to the merit of second strike capability. Some argue it's a great thing. Others argue that a Tap-Rack-Bang is the correct response. That's just a note, but perhaps we can start a new thread to discuss?

That's definetly a good way to say it short hand. I went with "DAO with reduced trigger pull" to keep it, hopefully, simple & approachable for folks who are new to guns.

I went with SFA because that's the term the Army and Air Force have used on recent RFPs and RFIs. I do agree with you that the examples you list illustrate the variety of ways the SFAs can seem to operate. However, the Springfield XD, as an example, is so far precocked that it's practically SA in operation. Yet the trigger pull must still finish cocking the striker the last little bit before releasing it. Technically it's still DAO by definition and by BATFE classification. Striker fired actions are just different and sometimes trying to describe.

Thanks to all who've contributed to the excellent discussion thus far. Please continue to expand and clarify the basic frame work of the thread.

 

Actually that's not even close. The Glock striker is about 60% pre-cocked. The connector cocks it the rest of the way and then cams down to release it. The BATFE defines Glocks as DAO. The SA XD is 95%+ pre-cocked and its connector does the same as the Glock's - still DAO according to the BATFE. S&W has a lightened DAO that requires the slide cycling to pre-cock it in a traditional hammer & firing pin setup. In both mechanisms the trigger is still performing double duty (finishing) cocking & releasing the hammer or striker.

 

I agree with that as well. However, as I stated in post 12, I'm trying to stick with current military terminology; right or wrong it tends to trickle into the civillian lexicon and get ironed out there.

Thank you for the encouraging words. My hope was to help shooters get a simplified view of how various guns operate. As with anything once the basics are grasped the details and technicalities can be discussed & debated. Hopefully this will help people to both understand how their handgun works, and clarify many of the terms discussed in the gun culture.

Thank you to all who've contributed thus far, and thanks for keeping this a great resource with civil discussion.

 

Mad Magyar, The Daewoo Fast-Action/Tri-Action is a truly unique mechanism. It doesn't really fit into any of the major categories because it can operate several ways. Perhaps you or someone with a good bit of experience with one would write a post explaining it. It deserves a good explanation because it's such a unique system.