I've read about the trigger spring "fixes" for the Sigma heavy pull weight, so I'd guess the stiffness is from heavy/long springs... That might not be it - I really don't know.

But, what about the really long trigger travel? I've never pulled a pistol apart (other than field stripping), so I don't know how everything works. Is the long travel the result of one or two parts? Maybe the geometry of how they work together?

Why are the triggers on some of the other DA/striker-fired pistols shorter and lighter?

I guess I'm curious.

The detailed answer on the various trigger designs is way too long for a post.
Reader's digest version is that the Sigma is "good" with the trigger designed to fire ANY ammo reliably, but has a trigger designed to emulate a revolver. It is the cheapest gun I recommend that is reliable for a defense pistol.
The SD (improved Sigma) is the "better" with the trigger designed with a different sear angle to provide a trigger for a self-defense gun, again with 100% reliability. I recommend it without reservation.
The M&P series provides the "Best" with a more sophisticated overall design, and different trigger options for different purposes. The service models are the LEO pistol of choice in many areas. I have an M&P 9 Pro with a factory trigger job providing shorter pulland only about 4# pull weight; it is designed for IDPA and USPSA stock production divisions. Can't go wrong if you have the cash.

Some combat tupperware like the XD have a different firing mechanism, and are classified as single-action because their strikers are fully cocked versus the partial cocking of the S&Ws. The service XDs retain a long moderate trigger pull, even though it is possible to make them much lighter and shorter as in the XDM.

If you want a 3# short single-action trigger pull, I suggest you buy an STI Edge competitiion gun and have it right out of the box.. Bring $$$$$$$. Such a trigger is not suitable for a defense gun.

It is simpler and more reliable to buy a platform that is nearest to your intended purpose and needs than to start with an entirely unsuitable gun and attempt to completely redesign it, and much safer, unless you are a specialiest that is fully competent in such areas. And the specialists I know will give you the same advice about starting with a gun already close to what you want.

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Science plus Art

Even shorter: the Sigma is a true double action (like a double action revolver) when you press the trigger you are compressing the striker spring from it's rest position to fully cocked (before it is released by the sear) though a 1:1 mechanical linkage.

Thanks for the responses.

Are others, like the Springfield XD or Glock or MP, not "true double action" pistols? I know I had read that they are striker-fired, but I also read that the Sigma was striker-fired. Are there differences in how this operation is performed? Something that makes the Sigma's pull so long?

Each design is a little different. For example the Glock has a "safe action", which has been classified as a double action trigger. The striker is not fully cocked until the end of the trigger pull, when it is immediately released. It cannot be cocked without firing and the striker has a "rest" position between contact with the primer and fully cocked. Thus for legal purposes it is classified as double action. Some will argue that Glock is not a true DOA. Whatever. The Glock trigger mechanism is the ultimate in polymer pistol mechanics and safety.

On an XD, the gun will be cocked after the slide is operated. You can see the cocked indicator on the rear of the slide. The striker is fully charged and waiting for your trigger pull to release.

Your fix to the Sigma trigger is the buy a better gun. There are much better options out there for not much more money.

Who says the trigger needs to be "fixed"? It works as it was designed to work in the absence of external safeties. And the only other pistols that are truly close to the cost are Keltecs or milsurps, some of those with even worse trigger pulls.

The question was asked because I'm wanting to learn about the differences. I'd like to understand how parts work together to create longer or shorter travel, lighter or heavier pull, etc. The mechanics - the levers, the springs, the pivots, the pins; the length of the internal components; how they are all related; their impact on a trigger's feel; etc. - I find that stuff interesting.

I'm sure the engineers/designers know what the heck they're doing, and the feel of a particular pistol's trigger is purposeful. I get that the Sigma's trigger was designed to be long and heavy, I'm just not sure how everything works together to achieve that.

Thanks, to everyone, for the replies. I'm kind of curious about the XD now...

The XD has a distinct trigger feel in my experience. A heavier but very distinct final application of pressure before the break, thanks to the striker already being cocked. With a Glock, especially in one without a trigger job, you can feel the striker cocking and all of the safeties being disengaged throughout the trigger pull. Neither is better or worse, it just comes down to personal preference.

The Sigma's DAO is simply another option. Personally, I like it. If you don't, there are plenty of options available including the SD & MP pistols from Smith. It generally makes more sense to simply change platforms than try to totally re-invent what you already have.

This video helped me understand better how the trigger sysytem of the Sigma works. I'm certainly no expert, but my guess after watching the video is that mechanically there are at least two things that contribute to the weight of the trigger pull on the Sigma. In my mind, they are the strength of the two springs in the sear housing block assembly, and friction among the various components (including but not limited to the trigger bar, sear, and striker). There are probably other factors at work there that I don't understand.


YouTube - S&W Sigma Trigger Modification part 1

My guess is that your two best ways to find out the truth on why mechanically the trigger pull is so heavy is to either talk to a pistolsmith who is experienced with the Sigma, or to find the S&W Sigma armorers' manual, assuming one exists.

I am in no way endorsing the modifications done in the video. I have no idea if they do or do not cause problems. I included the link to the video only because it offers a look at how the sear housing block assembly works.

Thanks. The video really did a nice job showing the springs responsible for the tension.

Again... not really asking because of any desire to mod my Sigma. I have a 5906 with a great trigger - especially in single-action mode - so I don't need to make my Sigma anything it isn't. Just trying to understand how everything works.

Maybe I'll do a little more digging to try and figure what makes the length of pull is so long.

Appreciate everyone's input.

The length of the pull is necessitated by the fact that the striker must be withdrawn far enough, against spring pressure, to insure that it strikes not just 'a' primer--but any primer--with enough impetus to guarantee ignition, even under adverse conditions.

To a degree this also affects the weight of the pull, since a spring of sufficient strength is needed to insure reliable ignition, again under adverse conditions.

Hope that helps.

Thank you. After thoroughly studying my Sigma's inner workings this afternoon, that makes a lot of sense.

I don't have a M&P or XD to play with or pull apart... do they have less trigger travel than the Sigma? If so, how do they get away with the shorter travel and lighter pull? I read something about one of them (I think the XD) having the striker in sort of a half-cocked position, but I have no idea what that would look like or how that would work.

I will pass on what i did with my sigma 9mm which caused a 5lb trigger and a pull of less than 1/8 inch. I pulled out the pigtail in the back just like all the trigger jobs say to do then i pulled apart the rest of the assembly and took out both springs and replaced it with a bic pen spring that i shortened just a little bit. Most people put the smaller spring back inside the larger spring but i only put the shortened bic spring in and have not had any problems with it at all. I also smoothed all of the parts. Part of the reason it is so heavy is because the firing pin diconnector needs to slide down to let go of the firing pin so smoothing that out helps a lot. This is also the reason the lighter striker spring from wolf helps because it is putting less forward pressure on that disconnector and lets it move down easier.

The XD is essentially a cocked single-action with a grip safety. The striker is maintained further to the rear during its 'ready' state and thus requires less travel to fire the piece.

I have no experience with the MP but I do know there is already an aftermarket sear available for it which supposedly 'helps' the trigger pull.

Like most other folks, I would prefer that my Sigma had about a five pound, smooth pull. I have however used a pistol (8 lb Glock) to save my hide on occasion and I didn't notice the pull weight at all. This is also why I won't tinker with springs, etc. in the Sigma. To me, the gun simply must work.