The necessary parts and tools are listed below:
Gas block and setscrews
1/8-inch Allen wrench
5/64-inch roll-pin punch
No. 47 drill bit
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.
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.