AR-MPR: Accurizing the Upper Receiver – Overview

A while back, I posted some AR-MPR rifle accurizing plans. This was my first-pass at enhancing the accuracy of the AR-MPR rifle. Since this post, I have had a few range visits (sorry, don’t have photos as the range does not permit photography) where I did not see the kind of accuracy and consistency that I experienced during the first range visit.

There are many factors that can affect accuracy for an AR-15 rifle, and one of them is the fit between the upper receiver and the lower receiver. This is (like anything else regarding rifle accuracy) quite controversial – but if there’s a bunch of slop between the upper and lower, you’re bound to get less consistent results. Based on the amount of slop present on this rifle, I decided to experiment with some techniques that I have either read about or devised on my own to tighten up the upper and lower.

Truing the face of the upper receiver on a metal lathe - Image Copyright 2011 Ultimate Reloader

So how did I decide that there was too much slop? Well, I started with a basic rocking of the upper assembly, and felt (qualitatively) that there was just too much slop. Following this, I dug in deeper to take some measurements. Measuring the channels for the lugs in the lower receiver, I found they were .502″ wide. The lugs on the upper receiver however measured about .495″ for the front, and .485″ in the back. That meant about .007″ of slop in the front, and .017″ of slop in the rear! Even an Accuwedge won’t fix that! In addition, there was a noticeable gap between the upper and lower- meaning that the holes in the lugs were too far “down” vertically to allow for a tight fit.

The concept I had was to modify the lugs so that they would fit the upper and lower receiver tighter together. I have a MIG welder setup for aluminum welding, a milling machine, and a metal lathe. What could go wrong? 🙂

In following posts, I’ll walk through the procedures that I undertook to tighten the upper and lower receiver, summarize results, and talk about what worked and what did not. While I was at it, I decided to true the face of the upper receiver that mates up with the barrel extension – see the above image. (This included making a custom mandrel for the metal lathe for upper receiver turning).

Stay tuned!


5 thoughts on “AR-MPR: Accurizing the Upper Receiver – Overview”

  1. There are certainly arguments on both sides of this problem as to whether or not this is actually a problem with accuracy. Most competition shooters I know say it isn’t really a problem. That said, some of the same shooters take the slop out anyway. I’m of the school that doesn’t like the slop. If for no other reason than a solid weapon gives me more confidence.

    There are a several inexpensive fixes that significantly improve this fit in most cases. The one I like the most is a set screw that goes through the lower receiver and touches the rear lug on the upper receiver. The down side is that the grim either needs an extra hole or must be removed to make the adjustment. The good new is that once it is adjusted it tends to stay right where needed. I use a rubber/plastic tip screw when doing this so there is actually no metal on metal interference. It also allows me to make a bit on the tight side as it will compress slightly when closed.

    A couple of cheaper or less time consuming fixes that are frequently used are:

    1- A thin O-Ring installed around the front lug prior to assembly, it’s amazing how well this .50 cent fix works. It takes all of the rocking motion out that exists between the upper and lower and can be installed in less than a minute.

    2- An AccuWedge. This piece is a retail product and actually does the same thing that the previously mentioned set screw does. It fits in the space between the upper receiver lug and the lower receiver well. There are of course two styles of lower receivers, top shelf and lower shelf. Depending on which of these designs you might have will determine whether or not the AccuWedge needs to be trimmed. On some top shelf uppers the AccuWedge will be trimmed almost razor thin on the lower portion of the product. I trim these with a razor blade.

    Both of these fixes will typically cure the problem.


  2. I run a JP rear tension screw and it has worked out well, although I haven’t tried any other “fix” as I doubt my AR-15 can shoot any better than it is now…

  3. Another cheap fix for taking up some of the “slop” between the upper and lower receiver is to place a foam earplug between the rear lug and the lower. What shooter doesn’t have a foam earplug laying around? It all comes down to whether or not you want to have the ability to take your rifle apart with or without a tool. If your using the rifle for target/ recreation then tighten it up. If you may have your life depending on your weapon at some point then a loose fit may be beneficial. This weapon system was designed with a little “play” between the upper and lower so it could easily be field stripped without the use of tools.

  4. I like the nut you fabricated for your quick-change tool holder. I thought I’d let you know I going to copy it for myself. 🙂

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