NM Rifle Build Part 10

In Part 1, we introduced the series and talked about tools.
In Part 2, we installed the magazine catch and trigger guard.
In Part 3, we installed and adjusted the Jewell Match trigger.
In Part 4, we installed the safety selector, pistol grip and bolt catch.
in Part 5, we installed the pivot pin, recoil buffer components and buttstock.
In Part 6, we installed the forward assist and the ejection port cover.
In Part 7, we installed the delta ring and barrel.
In Part 8, we installed the free float tube and front sight base.
In Part 9, we installed the handguards, flash hider, bolt carrier group and mated the receivers.

This is the final of the assembly posts. I didn’t specifically try to keep the assembly posts to an even 10, but it is convenient that it worked out that way.

The Front Sight base that came with my DPMS barrel actually had a sight post installed out of the box. It really would have been an acceptable front sight for a NM rifle as it is thinner than the standard A2 Sight, however, the Rock River NM rear sight I bought also came with an actual National match front sight post that I wanted to install.

What?

What’s the difference? I’m glad you asked.

I actually shouldn’t have labeled the one that came on the rifle as “standard” because the standard A2 sight post is a square .070″ post.

The one that came stock in the DPMS sight base was a square post, but significantly smaller than .070″…probably .050″ but I didn’t measure.

The National match post (shown in two different views, side and end-on) is not square, but rectangular. Its also beveled toward the front and is theoretically precision machined to create sharp, distinct edges. The shape is to cut glare and make a nice, clean edged sight picture even at long range.

The only disadvantage to the NM style sight post is that it is designed to be oriented in a specific way. That means when you adjust it, you have to turn it in or out a full turn to keep it oriented correctly, whereas with a square post, you can turn it in quarter turn increments. In other words, the standard front post has four times the adjustment sensitivity of the NM post.

Realistically, though, you don’t adjust the front post for a perfect zero anyway…the front post is only adjusted to get your rear sight in the adjustment range you want…so that isn’t really much of a disadvantage at all.

Installation is the same no matter which type of sight post you choose.

The front sight post assembly consists of three parts (other than the sight base, of course).

The post itself, the detent spring and the detent.

Some match shooters include a set screw with the same diameter and thread pitch as the sight post shaft.

What they do is tap the front sight post hole in the FSB all the way through, and then install the set screw through the bottom. After adjusting the front sight, they tighten the set screw up against it to lock it into place and prevent any movement at all.

I may do that at some point, but I didn’t do it during this assembly process.

The front sight base has two holes in the top. The smaller, front hole is for the detent and the rear, larger, threaded hole is for the sight post.

The detent spring goes into the front hole.

then the detent goes over top of that, with the “nub” pointing up.

Then the post is screwed into the threaded hole.

When the post is screwed down far enough, you’ll have to push the detent down to continue screwing in the sight post.

They make a tool for this, but I don’t have one yet. I never thought I needed it, but after having screwed with the front sight as much as I have lately, I can definitely see that it would make my life easier. I plan to order one from MidwayUSA when I order the sling…maybe today: It’s payday…just gotta pay the bills and see where I stand financially to decide whether I can afford it this time or have to wait for my next check.

It’s hard to see in this picture, but when installed correctly, the National Match front post will be oriented so that the ramped top edge is angled down toward the muzzle end.

It will have to be adjusted to get the zero point of the rear sight where you want it, just remember to adjust the National Match front post in full turns to keep it oriented correctly.

The Rock River Arms National Match rear sight is outwardly almost identical to the standard sight, but there are some differences there as well.

The most obvious difference is that the aperture has a “hood” that is designed to cut down on glare.

The aperture inside the hood is significantly smaller than a standard aperture as well.

The standard A2 sight comes with two apertures, a larger one for close in or low light work and a smaller one for distance. I don’t know the sizes, but I know for sure that they are significantly larger than the .040″ match aperture.

The smaller aperture is not suitable for a defensive or combat rifle because it reduces your field of view to almost zero, but it is great for match shooters because the small aperture increases depth of field (helps keep the target more in focus, even when concentrating on the front sight) and makes it easier to focus clearly on the front sight…as well as just having a smaller area in which to center the front sight post, which can help cut down on tiny errors that can make dramatic differences in impact point at 600 yards.

Another physical difference is the elevation ring. On a standard A2, the elevation knob is actually two parts, the bottom elevation knob, and the top elevation index. They are held together by a set screw. Turning the elevation knob changes the elevation. Loosening the set screw and turning the elevation index, just moves the range index marks to match the sights setting with the effective range that the sight is set at.

The Elevation knob that came with my Rock River National Match sight is a single, solid piece. There are no range markings on the elevation knob at all.

Another obvious feature is that this single elevation knob can be used for 1/2 MOA or 1/4 MOA adjustments, depending on how it’s installed in the rifle. On one side of the knob are detent dimples spaced for 1/2 MOA adjustments.

On the flip side, the dimples are closer together and there are twice as many of them…for 1/4 MOA adjustment clicks.

The final difference is not visible at all unless the windage adjustment knob is removed. The windage adjustments are also set for 1/4 MOA adjustments.

Well made National Match sights tend to be manufactured and calibrated with greater precision than standard sights as well. On National Match sights, one click should be very close to exactly 1/4 MOA (calibrated for a 20″ barrel), whereas standard sights may be in the ballpark, but not necessarily exact.

The RRA rear sight assembly came pre-assembled as much as possible.

The windage knob, ball bearing and spring were already installed, as were the leaf spring and aperture.

The only parts that were loose were the ones that had to be mated with the upper as the sight was installed. Those were the elevation knob, the sight body and elevation detent ball bearings and detent ball bearing springs, the elevation spring, and the rear sight roll pin.

First, drop one of the detent ball bearing springs into the hole in the right front face of the sight base.

It’s a good idea to put a dab of gun grease on top of the spring. That will help hold the ball bearing in place as the sight base is installed.

Then drop the detent ball bearing into the hole on top of the spring and grease.

Next, drop the other detent spring into the lower receiver into the small hole toward the front of the well that the elevation knob goes into.

And put the remaining ball bearing into the hole and on top of the spring.

Next, slip the elevation knob into it’s well in the receiver. It should push the ball bearing down out of the way as it goes in. When properly positioned, the ball bearing will be pressed into one of the elevation adjustment “dimples” on the bottom of the elevation knob, snapping it into place.

Be sure to put the correct side down for the sensitivity of elevation adjustments you want. Since the detent ball is on the bottom, that’s the side of the elevation knob that will determine whether you get 1/2 MOA or 1/4 MOA clicks. In this case I want 1/4 MOA adjustments so I installed the elevation knob with the side up that has the lesser number of dimples.

With the elevation knob installed and the threaded hole of the knob centered in the hole in the reciever, CAREFULLY lower the sight body so that the threaded post goes through the hole in the receiver and mates with the threads of the elevation knob.

Be careful to press the sight body detent ball bearing against the rear flat surface of the carry handle to prevent it from dislodging and falling out of the sight base.

Hopefully, the grease will help hold it in place.

Carefully turn the elevation knob counter-clockwise (when looking at it from the top) to screw the sight base post down into the receiver. You should be able to clearly feel the “clicks” as the elevation knob dimples pass over the detent ball bearing. Be sure to start out slowly. You’ll be pressing the sight body against the spring tension of the sight body detent ball bearing spring, which will be trying to cant the sight body as it goes in. Be careful not to cross-thread the elevation knob as you get it started.

Screw the sight body all the way to the bottom.

At this point, if you haven’t done so already, remove the bolt carrier and charging handle, then flip the receiver upside down.

Inside the receiver, you will be able to see the hole in the bottom of the sight base post. Insert the elevation spring into this hole.

Here’s where things got a little tricky. You have to be able to compress the elevation spring up into the sight body so that the sight pin doesn’t pass through the spring, but under it (over it in this picture because the receiver is upside down).

The problem is that if you press the spring down with something solid like a pin punch, its in the way and you can’t install the pin.

You need something to push the spring down, but won’t interfere with the pin passing through the receiver and sight base post.

They do sell an A2 sight installation tool just for this purpose, but I felt the asking price was a bit high for what it was, I decided to wing it. I made a sight tool out of a popsicle stick.

I used a round jewelers file to file a notch in the top of the popsicle stick large enough for the pin to pass through.

Then I filed the sides down so that it would fit in the hole in the sight base post.

I ended up with a makeshift tool that looked like this.

Not perfectly straight, but close enough for government work.

I used the popsicle stick, aligned fore and aft so that the notch in the top was aligned with the side-to-side pin hole, to catch the edges of the elevation spring and compress it into the sight base.

Once I got the spring down far enough, I used a pin punch to pass over the spring and temporarily hold it in place.

The spring should be captured completely by the pin punch. You don’t want the punch passing through any of the spring loops.

At that point, it was just a matter of driving the sight base pin in and pushing the pin punch out as it went.

And that’s it.

As usual, my explanations were more complicated than the installation procedure.

The last thing to install to make a complete, operational rifle is the magazine.

I purchased two, mil-standard 20 round magazines from MidwayUSA. The magazines are made by C-products, which I’ve heard good things about, but I only bought two for now so I can test them out and make sure they function well before buying more.

I did have one failure to feed and I’ve had the bolt fail to lock back on an empty mag a couple of times, so the jury is still out on these.

Right now, I’m still using cheapo PMC 55 grain ball ammo…which I’ve heard of people having feeding problems with. I’ll know more after I get some good match ammo and try it.

So, over the course of maybe 6 hours over two days, and ten blog posts we went from this:

To this:

Of course, this won’t be the last post about my new National Match AR, I’ve still got a few accessories to add (buttstock weights, possibly handguard weights, and sling), range reports to do, sight adjustments and zeroing, etc…but if all you were here for was the actual assembly process, this is the last of that series.

Click here for first range report

As usual, it took orders of magnitude longer to write the posts documenting the work than it did to actually perform the work…even with staging and taking pictures during the process.

As I mentioned before, my son and I did this assembly (actually, he did most of the work because he wanted the experience…I just took pictures and provided a helping hand from time to time), in about 5 hours on Saturday and about another hour or so on Sunday. We could have done it in half that had we not been worrying about setting up, staging and taking pictures and, now that we’ve done it once and know what we’re doing, could probably come close to cutting that in half again. That’s the beauty of the AR platform: it’s so modular and interchangeable that virtually anyone can assemble, customize and maintain them and the sheer variety of parts and customizations available is staggering.

I’ve already had a couple of comments on posts in this series from people who found them useful and I really appreciate that. Helping people by sharing my experiences is what these posts are all about so, if you find them useful, please let me know in the comments so I’ll be motivated to keep doing them as I experiment, practice and learn more about this gunsmithing stuff.

And, finally, to sum it all up, one more look at the cheesy little video I posted before starting this assembly post series:

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4 thoughts on “NM Rifle Build Part 10

  1. Pingback: National Match AR Range Report | Captain of a Crew of One

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