PMACA Chassis w/Ruger RCVR & KIDD Rear Tang Info - RimfireCentral.com Forums

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Old 06-30-2020, 02:57 PM
azguy
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PMACA Chassis w/Ruger RCVR & KIDD Rear Tang Info



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Recently I have been seeing more of these PMACA chassis. The request is for me to fit a Ruger OEM receiver with the KIDD Rear Tang. With that receiver then to be installed into the PMACA chassis. I have found a few details in doing these jobs that I feel y'all need to be made aware of.

The quality of the PMACA chassis is pretty good. There seems to be decent repeatability of inlet between them. So no issue on the chassis. The issue is when fitting a Ruger OEM receiver with the KIDD rear tang and then installing it into the chassis.

First off, this issue in this thread is not an issue insofar as the quality of the components goes. The issue happens in the fit up. The chassis inlet where the rear tang fits is true to the receiver inlet side rails. That is the tang shelf is parallel to the side shelf. Several issues to be aware of when fitting the OEM Ruger receiver with a rear tang and installing the Ruger receiver into the PMACA chassis properly.
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Old 06-30-2020, 03:10 PM
azguy
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And So It Begins

The first issue is the rear of the Ruger receiver. It is not perpendicular to the bottom of the receiver. Most of you have probably noticed this. I use a precision CNC vice as a "jig" when I mill the Kidd rear tang slot into the Ruger receivers. First making sure that the CNC vise is dialed in properly:




I mount the receiver in this trued up vise and then proceed. The bottom of the reciever rides tight against one jaw, insuring it is true. Then I checked the rear of the receiver to see just how much taper there is:




As you can see the rear of a Ruger OEM receiver is about .015+ out of parallel with the bottom of the receiver.
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Old 06-30-2020, 03:26 PM
azguy
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Now on a KIDD receiver the rear face is perpendicular to the bottom of the receiver. Since the rear tang rides on the face of the rear of the receiver, the bottom of the rear tang is parallel to the bottom of the receiver. So no issues there.

It would seem that the PMACA chassis was designed around the Kidd receiver, as the gap between the bottom of the rear tang and the bottom of the receiver mates up well with the chassis inlets. Which is great, if you're using a KIDD receiver.

There are a couple of major differences between the KIDD receiver with a rear tang and the Ruger receiver with a KIDD rear tang. The KIDD receiver is flat across the back of the receiver top. As we all know the Ruger receiver is radiused, or curved, across the back of the receiver. When fitting the KIDD Rear Tang, in order for the aesthetics to look good, I decided that the rear tang needs to ride even with this radius, at the corners of the rear tang.



Otherwise the rear tang corners would stick up around 1/32" or so, at those same corners. That would look horrendous.
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Old 06-30-2020, 03:28 PM
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typical Ruger QC
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Old 06-30-2020, 03:39 PM
azguy
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Since the rear of the Ruger receiver is angled relative to the bottom of the receiver, that means that the bottom of the rear tang will also be at an angle, relative to the bottom of the receiver. Now normally I would be bedding the rear tang and it's escutcheon into a stock. The bedding configuration, relative to the stock inlet, is then set by the rear tang itself. No problem, right?

But what happens when the shelf the rear tang will ride on, as to its angle relative to the receiver inlet, is already pre-set? In the case of the PMACA chassis that would be parallel to the receiver inlet side shelves. Here's what happens:




The first pic is a Ruger receiver with the rear tang installed, sitting in the PMACA chassis, loose. The second pic is the same receiver with the rear tang screw installed and snugged up. As you can see, the receiver tilts up at the barrel end. Why? Because the rear tang bottom is at an angle to the receiver bottom and the chassis receiver inlet. So back to the drawing board to figure out a solution.
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Old 06-30-2020, 03:57 PM
azguy
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And Even More!

There is another thing to take into consideration as well. Remember I set the rear tang such that the corners matched up pretty close to the radius on the receiver? We ll that also meant that the rear tang would sit lower on the receiver, relative to the bottom of the receiver.

A couple of solutions came to mind. I could mill the bottom of the rear tang to match that angle and set the gap to fit the chassis. Which would be a real pain. I decided to just take a clean-up cut on the rear of the Ruger receiver, just wide enough for the rear tang. This way the rear tang would be square to the receiver bottom. Then I would mic the relative chassis inlet depths, finding the position of the rear tang shelf, relative to the receiver inlet side shelves. The rear tang would be milled to fit that dimension.

Receiver clean up cut:


Modify the receiver to accommodate the rear tang and then install the rear tang. (I didn't take any pics of the rear tang modifications, my bad, sorry). Get the receiver set-up and dialed in so I can mill the bottom of the rear tang and receiver as needed:


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Old 06-30-2020, 04:09 PM
azguy
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And Even MORE More!

Milling the rear tang bottom:



Milling completed:


Keep in mind that the majority of the work is hidden by the chassis. There is a very slight silver line above the rear tang, at the top of the curve in the receiver. Just no avoiding that.

Check fit in the chassis, with just the rear tang screw snugged up. No rocking end to end at all.


Turned out pretty nice, if I must say so myself:


Thanks for checking this thread out. Hope it was helpful.

George
Sonoran Custom Gunstocks & Duplicating, LLC
[email protected]
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  #8  
Old 06-30-2020, 11:12 PM
Frank3

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George:

What about Tactical innovations receivers?

Also why not a hole that goes through the chassis where the rear pin is that holds the trigger group to the receiver and have anti walk pins [a pin that is threaded on both ends for small screws (AR15’s have them sometimes as anti-walk pins on the Fire control group)]?

This way you wouldn’t need to worry as much about the back end of the receiver unless the QC on the receivers is so bad that those holes are off also.


Any-who, the world looks awesome as usual


Sent from my iPhone using Tapatalk
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