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Posted: 11/26/2005 9:13:46 PM EDT
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Two-Stage trigger: 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 use - not counting those designed for competition. The KAC, and the RRA – the LMT is the same design as the RRA. The KAC is a simple, effective 2 trigger.  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. 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. Armalite trigger added 12/1/05  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, and I'm not easily impressed.  |
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Excellent post. So, to put this into perspective, in comparison to my gritty, single stage BM stock trigger, what's the operating differences? During the hammer release cycle, with a SS trigger the rear hook never comes into contact with the disconnector, correct? Also, is the front hook/sear engagement surface longer in length on a SS or are the angles simply different? |
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So, to put this into perspective, in comparison to my gritty, single stage BM stock trigger, what's the operating differences? >smoother pull, witha more crisp brake. During the hammer release cycle, with a SS trigger the rear hook never comes into contact with the disconnector, correct? >Correct. The SS trigger is just the second stage of a 2 stage trigger, with MUCH more sear engagement. Also, is the front hook/sear engagement surface longer in length on a SS or are the angles simply different? >I dont understand the question? 
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and Thanks
I believe you answered my question in your 2nd response when you said "...with MUCH more sear engagement". The gist of my question was if the "MUCH more enagement" was achieved through a different geometry (which requires more trigger pressure to achieve disengagement) or are the contact surfaces between the sear and hammer hook greater surface area (longer and/or wider) which I'm guessing would result in more friction/more pull weight? Also, after looking at my BM trigger, it appears the rear hook is positioned at the bottom of the hammer, not up on the hammer tail as you've depicted with the KAC and RR pictures. The only time the rear hook ever contacts anything is when it contacts the disconnector on the reset cycle. The hook for the sear is down way low on the hammer, and with everything installed, I'm not sure I can even see a sear. It looks to me like the front edge of the disconnector is being used as the sear and when the hammer hook engages that surface, it's almost directly under the hammer pin. But without taking everything apart, it's hard to see what goes on down in the bowels of the FCG. ETA: Sorry, I started posing my follow-up questions before the picture in your earlier response loaded on my screen. From the picture, it does look like the sear engagement surface is separate from the disconnector. |
For me to battle Mr Murphy, I replace the standard peened in place pin, with a long tool steel pin - I do the same with LMT triggers. Long enough that if it ever did come loose, it has no where to go. If your trigger isnt broken, dont fix it. 
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The Geissele trigger has a set screw for overtravel, sear engagement, and trigger weight, with a total of 8 pieces not counting hammer/trigger pins/springs. My initial thoughts where thats more screws than I want to worry about. I was really skeptical at first, but I'm still playing with mine to see what happens if they loosen to the point they come out. Give me a couple weeks to see what I can break on it, and I'll post a report in detail. |
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WOA Page with Another Good Pic of Giessele Where is the overtravel screw adjuster located, in the trigger housing or at the back of the bow? If it were to loosen, where would it end up, in the trigger's guts, maybe tying things up, or would it fall harmlessly to the ground? Also, Mr Giessele posted in another thread that his screw adjustments have some nylon, or a nylon-like layer coating the threads which makes them reasonably stiff in adjustment stock. He seemed to have pretty high confidence his screws would hold adjustment just fine. Maybe heat or solvents could affect them over time, who knows. If anyone has concern, just hit them with some Locktite as has been suggested in the past. ETA: Looks like it's maybe up front. Theres some adjustment sticking out below the trigger housing, just like on the KAC. |
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I dont want this to turn into a post about Giessele triggers. Give me a couple weeks and I'll have answers to everything about the Giessele, with all kinds of pictures. Just let me see how it holds up in my AR10 after some use. Yes, the overtravel screw is in the front. |
Sorry, My Jewell triggers are glued in place in my service rifle, and I'm not going to attempt to remove them and get them back in the same way. Jewell triggers have no place in this forum, IMO. They need to stay in the competition section. |
Nice pun, great thread. Can any of these triggers (or others, e.g. McCormick) provide, out of the box or through safe adjustment, a single stage pull with little creep, a 3-5 lb crisp break, and little overtravel, while remaining reliable enough for duty/service use? |
Not sure what you mean? You want to take a 2 stage trigger, and eliminate the second stage? I dont understand why people dont think a 2-stage trigger is reliable enough for duty/service use. This 2 stage trigger did just fine for many years, and few wars. 
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Excellent post, Steve -- well done once again! The Geissele is a good trigger. Bill re-designed his sear/disconnect geometry to fit both the AR/M16 and SR25/AR10T on USAMU request. It is a very fast, durable trigger. The AMU still uses Krieger/Milazzo triggers, however it's becoming very, VERY difficult to get replacement parts. We have the Geissele in our ammo/upper test lower receiver assembly; XM110 prototypes (AR-10Ts); and a few M4 Designated Marksman Carbines. |
I'm not suggesting that I believe there is a link between reliability and whether a trigger is designed to have 1 or 2 stages - I don't. However, I think some triggers are duty/service rugged while others are more suited for the range. I have inferred from your post that you feel the same (e.g. your comments on RR/LMT/KAC vs. Jewell). Away from the range, my personal preference is for single stage triggers. (I don't hate 2-stage triggers; I have some.) Do you know of any triggers that provide a single stage pull with little creep, a 3-5 lb crisp break, and little overtravel, while being reliable enough for duty/service use? I don't know the trigger market well enough to know if there are single-stage designs that provide this, or if there are 2-stage designs that can by design safely provide this via adjustment of the first stage pull length to zero. Thanks. |
In the RRA/LMT design, you can stone away some of the face of the disconnector. It will still disconnect, but when you pull through, there will be no second stage. I've seen several RRA triggers that people had worked on themselves, and didnt realize they turned their triggers into single stage triggers, yet the still function as a trigger. But, you still have the extra parts in the picture. Besides that, I dont have enough experience with single stage triggers, and actually I've heard of more fancy single stage triggers failing than, standard 2 stage triggers. |
Thanks. |
You should just call Milazzo and leave a message on his answering machine  Is that guy still alive? You'd think all his lawsuits would be enough to keep him going.If only I had money when I saw him on Commercial Row back when, with a table full of triggers. I think the only word I ever heard him say was "cash". I'm working on a write up of the Giessele, and its complex simplicity. |
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What a great, technical post. It helped so much to understand through your great pics and videos. Dr Mark, look into the McCormick single stage triggers. I have one for my carbine. Installation is a breeze (they really are drop-in), and the pull is 3 lbs. VEEERRRY smooth trigger to say the least. Just enough resistance for me. If you like single stage feel, then try McCormick. Good luck! |
I'm aware of the McCormick triggers, but haven't tried one yet. Since they're fairly new, I'd like to see how history judges them (how they hold up). |
Does the LMT trigger that i purchased from your website have this upgrade like the improved RRA trigger. I assume that it does as it was quite a bit more expensive than the RRA. Thanks |
Sorry, no. It cost more only because its "LMT". I'm sure the trigger is fine though, LMT uses a stronger pin than RRA. |
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