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New drop-in adjustable match trigger

This is a discussion on New drop-in adjustable match trigger within the Accuracy forums, part of the M14 M1A Forum category; I wanted to share progress on the new ShootingSight match trigger I am developing for the M-1/M-1A. I got my first prototypes done. There are ...


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Old October 8th, 2011, 10:15 PM   #1
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New drop-in adjustable match trigger

I wanted to share progress on the new ShootingSight match trigger I am developing for the M-1/M-1A. I got my first prototypes done. There are still a few minor design nits I want to pick, but it's close.

The trigger will be adjustable for pull weight and for second stage sear engagement without having to stone any sear edges. Sure, stoning can sharpen things up slightly, but the spec case hard depth is only 0.010", so stoning risks going through these, exposing the soft steel underneath, leaving it prone to wear. Hand stoning also has the disadvantage that it makes it difficult to get the sear edges even and parallel, so it takes a competent gunsmith to do it right. Granted, USGI triggers were not always straight, but at least they were machined using milling machines, so most should be close to straight.

Due to the adjustability, it is truly a drop-in trigger without the need to stone.

There are two components to my design.

First, the trigger has a hole drilled through it, so a spring that bears on the hammer spring housing can fit in there. This spring applies backward pressure on the trigger, countering the high hammer spring force.



(this prototype is a SAI cast trigger, but all others will be USGI or Beretta)

The actual spring design is still being worked, my first protptype uses a spring plunger. With this plunger set to max compression, first stage broke at 2-1/2 lb, second stage released at 4-3/4 lb. Not quite at the 4-1/2lb SR target, but a comfortable safety margin away.



Here it is installed, so you can see where it bears.



Here it is from the rear.




I am looking at using just a spring with a set screw behind it, to see if I can get the pull weight even lower. Disadvantage is you would have a separate spring, a pilot tip, and a set screw, so it is not as neat and compact as the spring plunger unit. Advantage would be that you could potentially get the weight even lower. TBD next week which design I like better.

The second part of the trigger is the adjustable disconnector. (I know this is called a sear in M-1 terminology, but the sear is technically the edge of the trigger that engages the hammer, I want to avoid confusion). This is an entierly new part, machined out of tool steel, and full depth hardened (not just surface hard). Typically, to adjust your second stage sear engagement, you stone the hammer edges to make the hooks shorter, so the hammer hooks contact the disconnector just before the trigger sear edges release the hammer. As discussed above, this risks going through the case hardened layer. Instead, I made a new disconnector that has a jack-screw in it. Screwing this forward will rock the disconnector back slightly, increasing the sear to disconnector distance slightly, rather than having to decrease the hammer hook dimension.

Two of my design nits here is that in the prototype, I put the screw hole a little too high. In the production version it will come out centered on the 'bump' in the disconnector. Second, I have not decided what to do about a screw. In the prototype, I used a headed screw, which limits you from screwing it in much too far. However, I want the screw tip to be domed, so it bears nicely on the trigger body. I have only found that option in a set screw.



Here, the trigger is at the point of second stage engagement, so you can see how little sear engagement remains when the second stage comes into play. The idea behind mnimizing 'creep' in a match trigger is to adjust the gap between the trigger sear and the disconnector, so it is just very slightly smaller than the hammer hooks. That way, when the hammer hits the disconnector, which provides the second stage spring pressure, the remaining sear engagement is minimal, but still safe. From this point, the additional trigger travel (creep) needed to release the hammer is so small the travel cannot be felt by the trigger finger.




The other change in the disconnector is that the top face is steeper than the original. This change is consistent with the USMC TM for NM trigger prep. I don't think it makes the trigger shoot any different, but at reset, there is slightly more overlap between the trigger sears and the hammer hooks, reducing the impact on the sear edges. TBD if I want to do this long term, or simply add the tapped hole to the existing disconnector to reduce costs. The existing part does have a 70 year history of performing.

Availability of the first batch will likely be late October.

Art

Thanks from a308garand, TREADMARKS, BDH and 8 others

Last edited by ShootingSight; October 9th, 2011 at 01:10 PM.
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Old October 8th, 2011, 10:28 PM   #2
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Art,

4 3/4 is perfect for match shooting though the rules states 4 1/2. I like your trigger, I remember how it was when you showed me your rifle at Perry. What I do recall whether I tried the reset or not. With your design is the reset affected as well?

I want one along with a high speed hammer.

Nez

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Old October 8th, 2011, 10:55 PM   #3
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Nez,

The reset is only a few thousnadths more - enough to add to reliability, but likely not enough for you to notice with your finger.

I'll put an announcement in the sales area, once I have production.

Thanks for the support.

Art

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Old October 8th, 2011, 11:29 PM   #4
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Excellent! I would be willing to give one a go. Did you ever consider giving Bill Geiselle a cal to see what he is using for set screws? Just food for thought. What do you think the min pull will be on the final version? I have a couple of scoped rifles I would like to try this out on.

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Old October 9th, 2011, 05:34 AM   #5
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Cant wait. I think you've already got a few customers in line.

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Old October 9th, 2011, 06:15 AM   #6
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The min pull on the spring plunger version is 4-3/4 lb. That's with the spring plunger in the full forward position. Min pull is limited by the travel of the spring plunger, once it is fully compressed into the plunger body, you can't go any further. If I go to a free spring design, I can reduce pull force further, but lower pull options with a free spring will take another month or two of development, since I still need to design, test, and produce the spring and two custom parts to make it work.

As I sit here and type this, I'm thinking I'll get my first batch out as Service Rifle triggers with the spring plunger design. It's very close to what you need for SR, it's based on a commercially available plunger which is a neat and clean package, so when you install it, you don't have parts trying to pop out and get lost, and its min pull weight is limited to avoid problems. I'll continue to look for new sources of plungers than might offer slightly more range.

In the next few months I'll continue development on a Match Rifle version, and see if I develop something I want to offer in the future.

Art


Last edited by ShootingSight; October 9th, 2011 at 06:29 AM.
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Old October 9th, 2011, 08:13 AM   #7
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I see you casted a NEW TRIGGER

Art7, I see that you stayed with a M14 type trigger in that the trigger pivit pin isn't fully suported. I think that M1 triggers use fully suported pivit pins, this adds some strenth in this area too prevent breakout, I also think it adds some extra leverage too reduce the pull weight in M14 trigger groups.

Worth a try?

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Old October 9th, 2011, 08:27 AM   #8
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good idea but you might want to bring the b.a.t.f.e. in on this and get an okay from them first. if some guy changed your screw out and went full auto on purpose, you might be liable. not positive but your primary engagement in the last pic looks very hairy.
i am a friend of bill akins. and that was not pretty. google akins accelerator.

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Old October 9th, 2011, 10:01 AM   #9
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Phil,

I had always heard that M-1 and M-14 triggers were the same ... my decision as to which to use will be based on supply. In looking at the cost of making new parts, the best way is to use GI surplus. Not so good is to use a cast part, which I don't love. So, if I find Garand or M-1 triggers will drive what I use.

M14xman. Good thought, and I'll follow up, but the reality is that this design is no different from every adjustable AR trigger on the market. They all have the ability to be adjusted to a range where the disconnector does not function properly. As far as I know, there is only one manufacturer out there that specifically uses a headed screw to limit that possibility. Now, if you want to protect against people intentionally modifying your product, consider that every stock trigger (including the Garand) could be made to do the same simply by removing the disconnector completely.

Not sure it can actually work on an M-1A, it might be that the timing is such that the hammer follows, but impacts the firing pin before the bolt is fully closed, so the safety bridge prevents ignition.

I appreciate your input, and will follow up, but my design is really not providing any risk that isn't already out there on every stock or adjustable trigger for someone intent on doing something illegal and dangerous.

Just to be clear - the primary engagement in the last picture IS very slight - because that picture is taken with the trigger pulled up to the point of second stage sear engagement, just so you can see how it adjusts the second stage sear engagement to be close to zero. The picture above it is with the trigger forward, and the sear fully engaged.


Last edited by ShootingSight; October 9th, 2011 at 01:26 PM.
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Old October 9th, 2011, 03:48 PM   #10
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Wow this is perfect timing. I just completed a M25 build and my only complaint is the USGI trigger. I'll keep an eye on the PX

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Old October 10th, 2011, 05:24 PM   #11
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Last of the new disconnectors will be finished tomorrow, and head off for heat treat, then black oxide. By about the weekend I should have the first ones assembled.

I won't yet have packaging or web site done, but if you don't mind that, the first batch should start selling early next week.

Price TBD, after I add up everything I spent at the EDM shop, but I expect it will be in the $165 range.

Art

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Old October 10th, 2011, 05:50 PM   #12
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Quote:
Originally Posted by art7 View Post
Last of the new disconnectors will be finished tomorrow, and head off for heat treat, then black oxide. By about the weekend I should have the first ones assembled.

I won't yet have packaging or web site done, but if you don't mind that, the first batch should start selling early next week.

Price TBD, after I add up everything I spent at the EDM shop, but I expect it will be in the $165 range.

Art
I was going to say that it looked like the threads were done on an EDM. I had to have some threads ''burned '' into a piece of carbide last week to repair a progressive die that is used to stamp Beryllium copper. BC is nasty stuff!!!

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Old October 10th, 2011, 07:13 PM   #13
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Yeah, the original plan was to EDM a pilot hole and tap the threads, but between the case hard and the fact that the geometry prevented the threads from starting perpendicular to a surface, EDM was the only way to do it.

I did some more playing with the prototype tonight and found out how to get the pull right down to 4-1/2 lb. This is gonna be a sweet trigger!

Art

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Old October 10th, 2011, 09:38 PM   #14
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Originally Posted by art7 View Post
... but at reset, there is slightly more overlap between the trigger sears and the hammer hooks, reducing the impact on the sear edges...
Could this additional overlap be used to salvage a previously overly stoned hammer (rear surfaces)?

That might be useful, in that I've a low milage M14 hammer that's been tweaked a little much for a second stage. (I don't have any idea from whence it came, it's just a random spare.) If so, it'll likely be a victim for the lightening tests...

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Old October 10th, 2011, 11:28 PM   #15
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It's easy to salvage your hammer - sort of. By making an adjustment to the trigger.

Look where the 'nose' of your disconnector touches the back of the TRIGGER body. Take some material off the back of the TRIGGER body there. This lets the discon swing further forward and closes the gap between the discon and the trigger sear, so it will still work with a short hammer spur. If it is just a tweak, put sand paper between the discon and the TRIGGER body, with the grit on the trigger body side, let the discon put pressure on it, and pull it through.

If you need more, you might try to either rock back the discon and sneak a dremmel abrasive disk in there, or else remove the discon, and touch the trigger to a belt sander or grinding wheel.

All of this will let the discon swing futher forward, decreasing the gap between the discon and the trigger sear, so if you have a hammer with short hooks, it will still catch it. Note that you do NOT want to take material off the disconnector 'nose', because that is the edge that needs to move forward in order to catch the hammer.

That said, if they stoned too far on the hammer hooks, they might have gone through the case hardened layer, and the mild steel underneath will wear. So even if you restore the gap, it might not stay as a consistent performer for very long before the tips will round. Net, it might be a lost cause unless you have it re-case hardened.


Last edited by ShootingSight; October 14th, 2011 at 04:16 AM.
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