A Catholic Mechanic: February 2020
Using barrel blocks or poly vise jaws, clamp your barrel in a vise and level the receiver. A note before I start: This procedure will work with a new FSB and one that has previously been drilled and pinned to a barrel. There should be a tab or latch near the base of the handle that you can press with a flat screwdriver and then the handle will slide off of the post. As seen in the picture below, you can also use the eyeball method here by sighting from the top of the level to the little flat machined area between the rabbit ears where your front sight post screws in. Doing your best to keep the receiver level (flat top clocked at noon) sight down the assembly and see if it is straight. I am fully aware that drilling and pinning the FSB is the best way to install it. You best bet is to file a new groove into the screw head, whether it be with a Dremel or hand file. You’ll have to be careful with the short one since there is very little thread engagement with this one but you can torque the long screw pretty firmly.
Snug your long set screw a little tighter and check alignment again. Whether you’re trying to repair door hinge screws or just a loose board, this little trick will fix a stripped screw in a hurry. Now tighten the muzzle device a bit more and this should provide enough pressure that you HG cap will not be loose when you are done. The A2 is a bit different since it kind of hangs out there and it susceptible to the accidental bumps and bruises of normal use. The pins will need to be installed both perpendicular to the barrel and square to the FSB so the pins come out on the opposite side at the same place they went in to put it simply. Then you will be able to get to the screw at 90 degrees. I used dial calipers to get “close enough”by splitting the width. If you have been able to keep the FSB tight against the handguard cap during the alignment/clamping process, it will generally be tight enough that it won’t rotate when you are done. I realize that the cap can only turn far enough before it hits the gas tube but a loose cap is very annoying nonetheless.
At that point, a shim could be added to tighten it up but to avoid all of this I came up with a method to insure your HG cap will not be loose behind the FSB. But I did have one failure and the HG cap was not as tight as I wanted when I was done. My only initial concern was that low-pro gas blocks are generally protected from exterior forces since many times they are hiding under a free float hand guard and only have to endure heat and vibration factors. If it is slotted you may be able rotate base to get it to come free from screw head. It is quite a difficult bugger to get a straight hold on, since it is tapered from top to bottom. The bottom line is- no matter what your table top and your level told you, it has to look straight.
Tweak it until it is and then cross check by putting your level back on the receiver. Now move your level under the FSB on the flat machined area of the bayonet lug. 21 drill bit. Then, drill the hole in the bayonet lug area and then enlarge the hole between the sling ears. Snug it a bit. Now install the other set screw, also with Loctite into the other hole and snug it. Snug it and check your alignment. Check your alignment again and then tighten both set screws alternately until you are comfortable with how tight they are. Pick up the assembly and check your alignment. You may have to loosen the clamp and set screw holding the FSB in place during the alignment process. Now re-tighten your clamp and set screw and re-check your FSB alignment. Now eyeball it. Does it look straight? If it’s still straight you are almost ready to start drilling. Now start snugging them up by work from side to side, diagonally.
If you push too hard it will try to rock side to side. Now I understand most of you will not be installing an FSB on a 7.5″ barrel. That would be an oddity (but not unheard of!) All that needs to be done is to cut a custom spacer depending on your barrel length/gas system type. Shown below is a spacer added to a 16″ barrel with a mid-length gas system. After buying a stripped barrel for an AR15 build I’m working on, I had to decide which type of front sight base/gas block I would choose to use. They do make jigs for the purpose of aligning the front sight base to the to the barrel extension index pin but it was up around a couple of hundred dollars to purchase. Drill and pin the second location. I would not recommend using the nearest fractional drill. By using aggregated Google search data they can determine what areas of the country are experiencing flu symptoms, “up to two weeks faster than traditional systems”.