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.
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).