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Old 10-05-2017, 10:33 AM
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Making a Decocker



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Chamber dings are bad news! Firing pins can (and do) break, stop pins can also bend/break, or they can possibly fall out while the bolt is removed. Even dry wall anchors or other types of snap-caps can fail. This protection is not fool proof.

The day I picked up my first Mark pistol, my FFL also delivered an old AR-7 I'd purchased. The AR-7 doesn't have a stop pin and the firing pin is driven into the breech with every dry-fire. Chamber damage is minimized by an extra tall firing pin that spreads the force of the blow over a comparatively large area. But damage still accumulates. Then I discovered that the AR-7 has one feature that is superior to the Mark pistols. It has a built-in decocker!

It works like this: If you take a cocked AR-7 with the safety off and pull the bolt back until it just touches the hammer, you can pull and hold the trigger and ease the bolt closed. This lets the hammer gently ride the bolt down and take the main spring pressure off the hammer - without needing to dry fire.

So, using the AR-7 decocker as a model, I developed a decocker for my Ruger MKIII. In extensive testing for the last 8 weeks, it's been working awesome!

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WARNING: This is a permanent modification. This information is intended for persons talented and brave enough to go beyond just swapping out parts to make their guns better. It requires a commitment to live with the new feature going forward.
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Initially, I thought about just posting the model for people to use, but then figured that most people are not likely to use what they don't understand. So instead, I'm posting the entire I process used to arrive at the model.

Feasibility:

The first question that needs to be answered with adding a new feature to a gun is: Can this mod be made with a reasonably good chance of success?

To answer that, we need to first understand how the gun functions now. The reason the AR-7 can decock is because it's trigger is only disconnected when the bolt is almost all the way back. So pulling the trigger when the bolt is just touching the hammer provides the desired result. On the other hand, the Mark pistol's trigger is disconnected ALL the time, except when the bolt is almost all the way forward. This would need to change. Let's take a look at the Ruger bolt and disconnector.



You can see that the "battery notch" Ruger puts in their bolts is quite large. There's no way I'm willing to mill such a large notch into my bolt. If the "decocker notch" can be made pretty small, I'm willing to take a chance that it will work without creating any problems.

In looking at the disconnector, it sure appears that the "battery notch" is made WAY bigger then it needs to be to allow for trigger function. I believe Ruger probably started with a much smaller notch, then expanded it as a way to reduce the bolt weight for proper cycling. We want the new notch to be as small as possible. It needs to be just big enough to allow the disconnector to engage the sear, plus enough margin so the hammer notch can rotate past the sear shelf. This seems feasible.

Starting the Design:

Obviously, the new notch needs to be located somewhere ahead of the "battery notch". But we can't exactly just put the bolt against the cocked hammer and mark the disconnector location directly. We need a creative method to determine the correct location. Also, we need to determine the length and depth of the new notch. So let's start with determining the depth.

The absolute most the disconnector can move up and down is the difference between the bushing diameter and the disconnector hole, as shown here.



We know that there has to be some clearance and tolerance margin built into this 0.100" swing. So I'm thinking about 50% will get us close and we may need to go as high as 80%. So start small and increase gradually is my plan.

Next, let's find where the notch needs to be located.

The only way I can think of to determine this is to pick a reference point on the bolt and measure the offset to the desired location. The first step is to remove the recoil spring. This will allow the bolt to sit in place while we mark locations on it; and it also makes it easier to feel the disconnector as the bolt moves back just into contact.

I chose to use small strips of electrical tape to do the marking, and the back of the receiver to align the strips to.



The back mark is where the bolt just comes in contact with the disconnector while it sits in the "battery notch". The front mark is where the bolt just makes contact with the face of the cocked hammer. It's important to note that the front piece of tape cannot sit across the opening as shown if the main spring is installed. You would need to mark the side(s) of the bolt stop pin opening. I manually held the hammer up in the cocked position when I made my mark.

Then remove the bolt and measure the difference.



The actual measurement on my MKIII bolt was 0.597", and was 0.590" on my MKII bolt. So the 0.600" is just rounded up. A few thousandths extra margin won't hurt a thing. We now have the offset.

The top/front corner of the "battery notch" is the reference to add the offset to. I marked the side of the bolt to show the front of the new notch's "milling zone".



Milling any further ahead then this won't buy us anything. This is because the bolt will be holding the hammer back, and enabling the trigger there won't add any additional functionality.

In looking at this 0.600" section of bolt, I decided that I really wanted to maintain at least 0.200" untouched next to the "battery notch". So I added the back mark for the "milling zone" there.

So here's the preliminary design:



Could it work?

Well, as shown so far - probably not. The reason should be apparent with this next picture.



There's not enough margin. We need to have ramps into both ends of the notch, and enough slack on the "battery notch" end to let the hammer notch rotate past the sear shelf. The disconnector barely fits now.

So, we have to make a choice. The notch needs to be made longer, or the disconnector needs to be made smaller.

Well, it appears that there is plenty of extra material on the back side of the disconnector that could be removed without causing any problems. This is shown in the orange triangle.

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SAFETY WARNING: do NOT remove any material from the front side of the disconnector, or make it any shorter!!! The front side works with the "battery notch" to limit the danger of an Out of Battery discharge. The height is set to ensure that the sear disconnects and prevents the gun from going Full Auto.
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I chose to make my disconnector smaller.

Making the Mod:

So before proceeding, what are the risks?

As far as getting the decock function to work - I see very little risk. The real risk is that the normal shooting function could be compromised, or that the bolt or disconnector could become damaged. So I looked at the risks.

The only possible risk I could see to the normal shooting function would be if the new notch would catch on the receiver and mess up the cycling. I examined this area closely and determined that there was enough surrounding support material to preclude this possibility. Green light there.

The bolt is quite hard and receives a LOT harder impacts from the disconnector at firing, and from the bolt stop pin at the end of backward travel. I think the bolt would be fine.

The disconnector gets hit quite hard by the bolt at firing and seems to survive it okay. But the risk would be if the higher velocity bolt could ding the top of the disconnector as the "decocker notch" passes over.

I don't think there's enough time for the disconnector to move up at all during the bolt's travel back. The small notch size and the bolt's high velocity preclude that. But what about during it's forward travel? It's still moving fast, but not nearly as fast as it's travel back.

Well, the disconnector is held down by the sear - except when it's aligned with a notch AND the trigger is released. The slower velocity and small notch size should limit any possible damage to just the very tip of the disconnector. But that would be the place to watch. Worst case is - once the disconnector got worn down enough to replace, I may have to replace it and the bolt.

I'm sure not going to throw this design out there and wait for someone else to be the guinea pig. So I manned up and proceeded to modify my MKIII.

My modified MKIII bolt:


My modified VQ disconnector:


My MKIII and MKII bolts side-by-side:


As I milled the "decocker notch", I went in "baby steps" to see how shallow I could keep it. The first try that worked was 0.042" deep. But that only worked about 85-90%. At 0.045", it worked 100%. To ensure it's always going to be 100% reliable, I went ahead and deepened it to the 0.050" shown in the preliminary design.

As of today (10/3/17), I've cocked/decocked my pistol over 400 times without a failure. And I've fired around 1600 rounds with no detectable sign of wear or damage to any parts.


I think this mod's a WINNER!!!


P.S. I think the same basic design process could be used to make decockers for Buckmarks, Victories, and other great rimfires that need them. Please post up if anyone takes on the challenge!
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