Building a Heavy AR-15 Varmint Upper Part 2: Barrel Assembly
December 31, 2013
In the first part of this article we gathered a pile of cool parts and now it's time to start assembly. Assembling an AR is usually a piece of cake requiring no gunsmithing to speak of. This project will go slightly beyond mere assembly but is certainly doable without a lot of special tooling.
The first step in assembly is test fitting the barrel extension to the upper receiver. Ideally we want a press fit between these two components in enhanced accuracy guns. If the barrel drops into the receiver easily with a loose fit, it can be shimmed and/or glued.
In this case, the bore of the upper receiver measured .9988 inches and the barrel extension measured .9984 inches. Less than half a thousandth of an inch difference. The barrel seated with a light tap from a plastic hammer. We were lucky. Sometimes I will test fit several uppers to a barrel to find the closest match. A good fit enhances rigidity and accuracy.
Our next problem was the barrel nut. It didn't fit over the fat barrel. Remember we are using the EGW float tube, and the I.D. of the nut was 1.022 inches, which would normally fit most barrels but our special Rock River heavy varmint barrel is 1.051 inches.
The nut was set up in a lathe and a boring bar was used to carefully remove .040 inches of material. It fit perfectly. A drop of oil was applied to the receiver threads and the nut was tightened and loosened several times to seat the threads. Final tightening was accomplished with a PRI barrel nut wrench (Brownells 714-000-007) and a breaker bar.
The hole in the nut that the gas tube will pass through must be in perfect alignment with the hole in the upper receiver so the tube free floats. We check alignment with a #15 drill bit inserted into the gas key.
Slide the bolt carrier into the upper receiver, letting the drill bit pass through the gas tube hole. If the drill binds or doesn't pass through the hole, the position of the nut must be tweaked left or right until it does.
The next assembly step is the gas block. The gas block mounting surface of the barrel measures .936 inches and the inside diameter of our Troy low profile gas block is the same. By polishing the inside of the block with a flapper wheel in an electric drill, we loosened the fit slightly so it would just slide on to the barrel without beating it into position.
How are we going to index the block to the gas port? Some barrels provide a dimple 180 degrees from the gas port for indexing. Rock River didn't bother.
To create our own dimple, we first need to measure the distance from the barrel shoulder to the center of the gas port. I checked the port size with gauge pins and found it is .086 inches, so we can use the gauge pin or a #44 drill in the hole.
We measured with calipers from the barrel shoulder to the pin, added half the pin diameter and the final number (.288 inches) is the distance from the shoulder to the center of the port.
The barreled receiver is leveled upside down in the mill vise and the drill chuck is centered over the barrel and the barrel shoulder with an edge finder. A center drill is mounted in the chuck and we move the table over .288 inches and make a dimple.
The port in the gas block is almost twice the size of the barrel port, so a slight misalignment won't affect function. You could probably eyeball the location and get it close. I like them centered if possible.
Now the gas tube is installed in the block with the roll pin, the inside of the gas block and the gas block seat on the barrel are degreased and Loctite 620 Sleeve Retainer is applied to the barrel, leaving a gap around the port. We don't want to block the port.
Slide the gas block into place with a twisting motion to spread the Loctite and install the set screw through the bottom of the block into the dimple. The screw is really only used to locate the block correctly and the Loctite 620 will retain it. Release temperature for the Loctite is 450 degrees Fahrenheit and I don't see our heavy barrel ever getting anywhere close to that hot.
Stay tuned for part 3 of this series of articles on building a heavy varmint upper.