Trigger Mechanisms; A Guide to What's What

[ugaarguy]

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

[marzen]

Great write up and thanks for sharing!

[PX4MAN]

Thanks Ugaarguy, I'm going to print
this out and make some copies to just
hand out when somebody wants an
explanation and be correct and cover all.

john

[ugaarguy]

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

[rcellis]

Where does the Walther AS and QA triggers fit in? And the 'New York Police' trigger?

[ugaarguy]

Where does the Walther AS and QA triggers fit in?

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.

And the 'New York Police' trigger?

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.

[briansp82593]

dont forget lda also it must be cocked but when its cocked it protrudes only a few mm's so im thinking that would classify as sa? or i dont know

[PaladinX13]

For "Errata" you probably could make some mention of the new Taurus SA/DA trigger mechanism (I know it's confusing since Taurus recycles model names). And I'm not sure it really falls into the arena of triggers, but maybe a mention of things like the SFS Browning Hi-Power hammer?

[CJ]

For "Errata" you probably could make some mention of the new Taurus SA/DA trigger mechanism (I know it's confusing since Taurus recycles model names). And I'm not sure it really falls into the arena of triggers, but maybe a mention of things like the SFS Browning Hi-Power hammer?

Agree with PaladinX13.

SA/DA - Single Action/Double Action. Currently in use on some Taurus models. These pistols are designed to fire the all rounds Single Action. If there is a failure to fire, these pistols have the capability to fire in Double Action mode (referred to as second strike capability: allowing a second strike on a primer, frequently firing it on the second attempt).

[Soybomb]

DA/SA also goes by its older name of traditional double action, or TDA alot.

[default]

Overall, an excellent summation and on a subject I find very interesting. However -

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.

All of what you say here is true, but I would submit that the term "Quasi-DAO" might be preferable because it covers everything from a Para-Ordnance to a GLOCK 17 to an SIG P229 DAK. I guess I don't like the term "Striker-Fired Action" in principle because, for example, the Walther P99, which is striker-fired, is available in DA/SA and true DAO configurations. On the other hand, the striker-fired Browning 1910 is SAO. In other words, whether the pistol is hammer-fired or striker-fired is not necessarily relevant to the trigger mechanism, unless I'm missing something.

Having said that, and acknowledging ahead of time that I'm not an expert and may well have overlooked some important point, great post!

[ugaarguy]

dont forget lda also it must be cocked but when its cocked it protrudes only a few mm's so im thinking that would classify as sa? or i dont know

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

For "Errata" you probably could make some mention of the new Taurus SA/DA trigger mechanism (I know it's confusing since Taurus recycles model names). And I'm not sure it really falls into the arena of triggers, but maybe a mention of things like the SFS Browning Hi-Power hammer?

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

These pistols are designed to fire the all rounds Single Action. If there is a failure to fire, these pistols have the capability to fire in Double Action mode (referred to as second strike capability: allowing a second strike on a primer, frequently firing it on the second attempt).

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?

All of what you say here is true, but I would submit that the term "Quasi-DAO" might be preferable because it covers everything from a Para-Ordnance to a GLOCK 17 to an SIG P229 DAK.

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 guess I don't like the term "Striker-Fired Action" in principle because...

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.

[briansp82593]

i think glock's is quite unique, its striker fired but totally sa, when you pull the trigger all you do is release the striker, no da backup... so... sfsa striker fired single action, unlike other sf pistols that are cocked as the trigger is pulled, aka kahr and SAA

[ugaarguy]

i think glock's is quite unique, its striker fired but totally sa, when you pull the trigger all you do is release the striker, no da backup...

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.

[CypherNinja]

I've always thought of SA, DA/SA, DAO, etc..... as a completely separate issue from striker vs. hammer.

A hammer swings about an axis, while a striker moves back and forth in a linear fashion. Details on how each mechanism is cocked and released is irrelevant.

I think its confusing and inaccurate to try to lump all the striker designs into one category. (regardless of various marketing campaigns)

It should be "a striker is just a different type of hammer, and uses all the same types of trigger mechanisms listed above.

Just my .02

[DougB]

I agree with Cypherninja - striker vs. hammer is a completely seperate issue from trigger function. I have an SW99 (the original trigger, now called "anti-stress" by Walther). It is striker fired, but the trigger functions exactly as a traditional double action (DA/SA) gun with a hammer. The first shot is fired double action (no pre-cocking). After that, the gun is cocked (single action). This gun has a decocker button, and functionally works exactly like a traditional DA/SA (Walter P38, PPK, CZ75D, and many others). The fact that it uses a striker rather than a hammer is irrelevant to the trigger function.

Other than that, and the fact that several of the gun models used as examples could fit into multiple categories depending on the particular trigger options selected, I think this does a good job of summarizing information that is confusing to many new shooters.

Doug

[ugaarguy]

I agree with Cypherninja - striker vs. hammer is a completely seperate issue from trigger function.

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.

I think this does a good job of summarizing information that is confusing to many new shooters.

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.

[Kowboy]

Ugaarguy:

Thanks for the great read, now may I ask for more?

Are there any computer animated videos that would show these different actions? I'm kind of a visual guy.

Thanks again,

Kowboy

[JohnKSa]

Glock Animations:
http://www.genitron.com/glock23/intglock.html
http://www.sniperworld.com/glock/

Beretta 92FS Animation: (Sorry, there is no provision to demonstrate DA operation)
http://www.genitron.com/IntPistol.html

1911 Animation:
http://www.m1911.org/loader.swf
http://www.m1911.org/full_1911desc.htm (page has several animations)

[BrokenArrow]

The terms are getting harder to nail down as the technology advances. Hammer and striker fired guns can both be SAO, DAO, or DA/SA(TDA).

IIRC, the BATF called the HS/XD a SA pistol. Why IDPA puts in a different class than the Glock.

Federal and civilian RFP definitions are all over the place. Things changed as the Army's FHS merged w SOCOM's SOF CP into the JCP that went back to CP and was then indefinitely postponed... for example from the Q&As on the JCP RFP :

51. Item 3.4.1 Action. Will all Striker Fired Actions be considered Double Action Only regardless of what the striker does during the action of pulling the trigger? Is there any Double Action definition as to what makes it a DA or DA pull? How will Double Action be defined?

Answer: The specification will be changed to state: DA/SA is defined as an action that sets and releases the sear with the first pull of the trigger, with subsequent shots being single action, only releasing the sear from a preset position, to fire the weapon. DAO/ Striker Fire is defined as an action that sets and released the sear with every pull of the trigger. Note: Any Striker Fire action that fully sets the sear will be considered DA/SA.

63. What do you mean by "Modular Action" and what will be required to for it to be considered changed. Item 4.4.1 states that the JCP must be able to be able to be changed from DAO to DA/SA, and back. If this is the case, how does the striker fired guns fall into this category? If the gun must be reconfigured from "Striker-Fired Action" which is considered DAO to a DA/SA gun which requires both a cocking action trigger pull and a pre-cocked action trigger pull as well as a de-cocking lever and external safety lever. I would appreciate some clarification on this matter.

Answer: Modular means that the action on the pistol can be changed at the unit level without modification to the weapon’s major assemblies from DAO to DA/SA, or DA/SA to DAO and back.

How about this?

3.4.2. Trigger Pull: ...When pressure is applied to the JCP trigger and then released, the trigger shall reset to its forward-most position, even if the pistol is not fired (T). The operator shall be capable of pulling the trigger, without shifting the firing grip as will be tested in section 3.6.3.

What the heck did they mean by that? Could read it in a way that leaves the Glock and XD out in the cold (second strike? can't cycle slide to reset striker?)

The DHS in 2004 made it very simple:

3.2 Operation: The pistol shall be an action that provides smooth and consistent trigger pull for the first shot and all consecutive shots.

SIG(DAK), HK(LEM), Glock(SA), XD(USA), S&W(DAO), and Beretta(DAO) entered. Bertta withdrew early. When the smoke cleared after firing about 3 million rounds in 9/40/357 through about 300 test pistols, only SIG and HK got contract options for up to 65,000 pistols over 5 yrs.

Will be interesting to see how they define things for the next go in 2009, who enters, and who passes.

[mekender]

i believe that the new ruger is also a striker fired weapon

[polekitty]

Recently I bought a Taurus 24/7PRO. It has a SA/DA trigger system with “re-strike” capability. Frankly, I think I would not bet my life on that re-strike. If the cartridge failed to fire the first time it might do it again on the second try. I don’t think I want to keep hitting that “dead” cartridge over and over while the other guy is making holes in me! I’ll rack the slide ASAP and go for a new load!

[gunboat57]

One more comment on the Taurus 24/7 Pro. When firing "SA" the striker is held about 95% of the way back by a sear. Pulling the trigger pushes the sear rearward, compressing the striker spring a bit more while the sear is cammed down to release the striker. It's like a Glock with more precompression. It sorta blurs the distinction between DOA and SA/DA.

[Mad Magyar]

In what category do you place the Daewoo in "Fast-Action",a.k.a. "Tri-Action"?

[ugaarguy]

In what category do you place the Daewoo in "Fast-Action",a.k.a. "Tri-Action"?

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.