In my last post, outlined some ideas and goals for talking the slop out of the fit between the upper and lower receivers on the AR-MPR rifle. The first step is to remove the gaps between the sides of the upper receiver lugs, and the channels at the front and back of the lower receiver. With approximately .017″ of total slop (front gap + rear gap), that would equate to approximately 8″ of  hypothetical variation in point of impact (if the receiver could twist fully in each direction).                                .

While I contemplated bedding the lugs with the receiver with epoxy, I decided that since the lugs are bearing surfaces when the upper and lower are opened/closed, this could cause wear with a “softer” material like epoxy. I am looking for a more long-term fix.

The experiment:

Based on the goals, and the tools that I have on hand, I decided to remove the hard coating and weld the lug holes shut, leaving excess aluminum “raised up” that could be milled down to the proper dimensions. While not as ideal as hard coating, the aluminum should prove reasonably resistant to wear and tear.

Step 1: Prep the Surfaces

I did two things to remove the hardcoat (anodizing) from the lug surfaces where welding would take place. First, I filed down the sides of the lugs. I was amazed at just how hard the hardcoat treatment is. It felt more difficult than filing mild steel by far! Second, I drilled out the holes to remove the hardcoat from the insides of the holes. Again, I was amazed at how hard the harcoat treatment was to drill – in fact, I had to sharpen the HSS drill bit a couple times to get the job done. Drill Doctor to the rescue!

Rear lug prepped for welding - Image Copyright 2011 Ultimate Reloader

Step 2: Weld the Lugs

I’ll admit- using my Lincoln Powermig 200 for this job was a bit of overkill  – in fact, I would have preferred using a TIG welder (something I’ve yet to try) – but I thought this would be worth a try.

The welder - A customized Lincoln Powermig 200 - Image Copyright 2011 Ultimate Reloader

MIG welding aluminum? Don’t forget your spool gun! I used 100% argon gas and my spoolgun for this welding job. I don’t weld aluminum frequently, so I had to experiment on a test piece, and put on a split tip. Once I got things setup, it was time to go.

Aluminum welding spoolgun - Image Copyright 2011 Ultimate Reloader

Welding aluminum is very exacting- it’s not like welding steel where you have a high degree of flexibility on temperature, and can see the temperature by examining the color of the weld puddle (Red = Cold, Yellow = Hot). With aluminum, you have a response more like microwaving butter. All solid one second, melted through the next second. You really have to watch the shininess of the edges of the puddle, and practice a lot.

Overall, the welding went well using quick “blasts”. As I expected, the corners rounded off a bit due to the high intensity of heat.

The result was not perfect, but given the tools at hand, and the experimental nature of this project, I felt the outcome was acceptable. Next, we’ll cover milling the sides of the lugs, and re-drilling the holes.

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!

-Gavin