AR Upper-Receiver Build: Gas Block/Gas Tube Installation

Now that the barrel is in place, the next step you need to tackle is the gas the rifle's gas system.

The necessary parts and tools are listed below:

Parts

Gas tube
Gas-tube pin
Gas block and setscrews

Tools

Level
1/8-inch Allen wrench
5/64-inch roll-pin punch
No. 47 drill bit

Note: In my opinion, Towsley underemphasizes the importance of gas-tube orientation here. It tube needs to be oriented so the bleed hole that feeds into the gas block is at the 6-o'clock position, and the remainder of the tube angles upward into the receiver. I want to mention this up front, because this tidbit of information ultimately came back to bit me in the posterior, so double-check the position of the bleed hole, otherwise you'll have problems later. (Don't ask how I know this.)

 Towsley urges putting the gas tube in place and then threading the gas block in place before pinning it in place. Here's another alternative method suggested by Shooting Illustrated Rifle editor Steve Adelmann of Citizen Arms: Pin the gas block to the gas tube, as described above, then side it onto the barrel and then anchor it in place. (Keeping your bolt-carrier group installed during the process also helps keep the rearward portion of the gas centered as it goes through the receiver.)

During the installation process, go slowly so you don't scratch your barrel like I did. (There are some barrel nuts, however—such as the one that came with my Geissele Super Modular Rail MK4 Integrated M1913 Picatinny Fore-End—that don’t require the gas tube to be threaded through the barrel nut.) Slide the gas block down the barrel, being careful not to mar the finish.

Put a drop of No. 242 Loctite on the setscrews and install them in the gas block, but do not tighten. With the receiver clamped in the vise, level the upper assembly. Then place the level on the gas block rail, and turn the block until it is level and in perfect alignment with the rail on the receiver. (If you don't have a level, determine the center of your gas block/barrel and add scribe pencil or magic marker lines on each prior to anchoring everything.) Now tighten the setscrews.

If you want a more secure lockdown of the gas block, after lining it up correctly, remove the setscrews one at a time and drill a shallow dimple in the barrel for each screw to lock into. I don’t think this is necessary unless you are going to mount some heavy accessories on the rails of the gas block. If you do it, be careful to use a drill press with a stop so you do not drill too deep and ruin the barrel. Do one screw at a time, replacing each one before removing the next so that alignment is maintained.

Insert the gas tube into the gas block. Make sure the end of the gas tube lines up with with the three holes is in the gas block. Also make sure you have the large hole in the gas tube facing down. If you don’t, you have the fanciest-looking, single-shot rifle in town!

Start the gas tube pin in the gas block using a 5/64-inch roll pin holder punch. It’s a little tough to line up the hole inside the tube with the hole in the block unless you use a slave pin. The shank of a No. 47 drill bit is the right size for this job. After aligning the holes, carefully drive in the roll pin using the 5/64-inch roll-pin punch until it is at an equal depth on each side.

The next entry will cover installing the fore-end.