I pulled this from the archives and added a few more pics and better video.
Two-Stage trigger: A trigger which releases the hammer which makes the rifle go bang, after two distinct amounts of mechanical pressure are overcome by squeezing the trigger. The initial resistance, long and light(usually about ¼”) referred to as “take up” is the first stage. The second stage, offers much more resistance, but for a much shorter duration.
The initial “take-up” on the resistance of the trigger spring moves the sear almost completely out of engagement. This “take-up” ends when the rear hammer hook contacts the disconnector. Continuing to pull the trigger, the rear hammer hook will push back the spring loaded disconnector, until the front hammer hooks disengage the sear and the hammer strikes the firing pin. It’s the resistance of the disconnector which gives the trigger the heavy, second stage.
As the action cycles, while the trigger is still held back, the rear hammer hook, is caught by the disconnector, which releases the front hook into the sear when the trigger is released. The angle, and spring loaded pivoting of the disconnector ensure, the firearm fires 1 round with each pull of the trigger.
Sear Release Video
Disconnect Video
Simple enough?
Below are pictures of the most popular 2-stage triggers used in defensive carbines. The KAC, and the RRA – the LMT is exactly the same design as the RRA.
The KAC is a simple, effective 2 trigge.
Here is the KAC trigger, a simple design using the trigger pin to hold the sear/disconnector and trigger in place. The pictures position is cocked and ready to fire.
From this angle you can see the adjustment screws for weight and sear engagement. Sear Engagement is often referred to as “creep”. The less sear engagement, the less “felt creep” in the second stage of the trigger pull. The weight of the 2nd stage is more dependent on the disconnector spring. A finely adjusted sear engagement makes the “break” crisp, where you may think it’s actually lighter than it really is.
Here is the trigger with the first stage of pull taken up, with the final sear engagement shown in the close-up caption. The rear hammer hook is up against the disconnector on the left. As the resistance of the disconnector spring is overcome, complete sear disengagement will follow along with a “bang”.
The pieces of the KAC trigger – I mocked in the adjustment screws, but you get the point. It has more parts to fail than a standard trigger, but its pretty durable. Short of the sear or disconnector breaking, about the worst that can happen if the screws loosen, the trigger can get heavy, or become a single stage trigger.
The more economical/popular/common two-stage triggers are the RRA/LMT Pictured is a RRA, with a replacement disconnector pin I added for a bit of insurance.
This trigger is not easily adjusted, and should just be left alone. If you “need” to a finely tuned trigger, shell out the bucks for one that was made to be adjustable. This is good trigger, which is a vast improvement over the standard single stage AR trigger. The weak link in this design is the need for an additional pin to hold the disconnector in place.
Unlike the KAC, the sear is part of the trigger, and not an extra piece. The rest of the parts interact the same.
Here is the trigger with the first stage of pull taken up, with the final sear engagement shown in the close-up caption. The rear hammer hook is up against the disconnector on the left. As the resistance of the disconnector spring is overcome, complete sear disengagement will follow along with a “bang”.
The pieces of the RRA trigger – No screws to come loose, but if the stock disconnector pin comes out, you’re left with an 8lb club. The only way to properly lighten the RRA trigger is to replace the disconnector spring with a lighter one. Stoning the hammer hooks and disconnector only remove some of the surface hardening. It may be nice in the short term, but no promises down the road.
There is a new player on the block. The Geissele trigger is in a league of its own, and deserves a thread itself once I’m done with my T&E. If all the little pieces stay where they are supposed to, this trigger will be leaps and bounds beyond anything else out there today.
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Well, I have put over 20K rounds through my AR with that trigger picture above, and it still works as well as day one. There have been enough threads on the Hi-Speed in the last couple years that, what needs to be said has been said.
Keeping in the spirit of the original thread, I need to add a section for the Geissele SSA and SSF. Pictured below is the SSF. It’s the same functionally as the SSA, but with full auto capability which I’ll also cover. Sorry, I don’t have an SSA for pictures – actually I don’t have an SSA of my own at all – they are all going to back orders before I take one for myself.
The details of the SSA/SSF are covered
HERE We should note in comparison to the other triggers here, the simplicity and durability of the design compared those above.
I’m sure there are some here who don’t know how M16s function full automatic, and this is an appropriate topic to cover the topic.
The difference between the semi and full auto(this also covers standard M16 triggers) is the disconnector, selector, and the auto sear as shown below.
Here is the SSF and accompanying pieces for full auto function as they relate to each other.
The extra pin over the safety selector is the pin which is holding the auto sear in place.
In full auto, the sear attached to the trigger is bypassed, as the disconnector is held down out of the way of the hammer hook. The sear hook on the top of the hammer is caught by the auto sear instead. When the bolt closes and lock, the bolt carrier will trip the auto sear releasing the hammer as long as the trigger is held back.
In the picture below, you can see the cam on the safety selector, which when in the full auto position, keeps the disconnector depressed.
The M16 disconnector has a little tail so it is long enough to make contact with the selector. Note the opened back of the trigger and the disconnector with the selector removed.
The semi auto triggers have that protruding tail removed to insure unobstructed constant disconnection.
HEREis my latest youtube upload which covers the function in more detail, which I hop answers some other questions.