DIY A2 FSB Install- Version 2.0

@A2FSB

I called this blog post "Version 2.0" because I covered another FSB install process in a previous post- but that was with the set screw method.  DIY A2 FSB Set Screw Method

A question answered before I start: Why on earth would you attempt to drill and pin an A2 FSB on your barrel and take the risk of screwing up your barrel or ending up with a canted base when you can just send it away and have it done professionally?  Answer: The whole point of DIY is to avoid hiring out work and take pride in doing it yourself.  As I will mention in the backstory at the end, I like to tinker and I like a challenge. And lastly, I really wanted to "do it right" and avoid the set screw method. Although the set screw method is an alternative to the traditional drill/pin method (which is the industry standard for properly mounting an A2 FSB) it is not a replacement.

(This post is set up so that all commentary is ABOVE each image)

 

The "F" Mark 

Also before we start: FAQ- Should I use an "F" marked front sight base or not?

Here is the difference

I can't explain it better than Mrgunsngear, so take a minute to watch:

 And a link to Molon's article on AR15.com which is referenced in the above video.

The whole article is a great read and very informational. FSB info is at the end of the article if you want to skip to it.

AR15 Zeros and Trajectories

 Tool/Component List

2/0 Straight-Flute Hand Reamer 2990A16 McMaster Carr

#31 TiN Coated 135 Degree Point Drill Bit 8903A44 McMaster Carr

Drill Press (Required, bench top size will do)

Small Tap Wrench

Small Precision Level

Small "C" Clamp or Vise Grips

Means of Holding Barreled Upper (homemade jig in this case)

Previously Installed A2 Front Sight Base

Virgin A2 Front Sight Base

2 Taper Pins (Proper size is 2/0 X .845")

Stripped Barrel

Handguard Cap

Barrel Nut

Upper Receiver
Dial or Digital Calipers
Additional Tools/Components Not pictured:
Barrel Vise Blocks
# 21 drill bit
10-32 taper tap
#10-32 X 1/8" set screw

Resource list for buying tools and components is at the end of this post.

A Few Notes Before We Start

  If your barrel is surfaced treated (nitride, melonite, QPQ) OR you are using a .625" ID FSB, you might want to re-think the home install option. Consult a professional before proceeding. This procedure was done on a phosphate coated barrel with a .750" gas block area diameter.

Also- this process can be also be done on barrels which have been dimpled for a lo-pro gas block. The dimples should not affect the process. 

This procedure will not work on barrels which have been previously drilled/pinned for an A2 FSB. 

It is possible to install a virgin base on a barrel that has been previously drilled/pinned but it will require the tooling and expertise of a experienced machinist. 

          Step 1, Option 1: Determine Hole Location

I must say I "cheated" on this step. I had a previously installed FSB in the parts box so I figured I'd use it as a template for the new one. Set them up side by side then take your hand drill and make a little dimple in the surface. Doing this assumes that the previously installed FSB was installed correctly, of course. So do this at your own risk if you don't know the origin of the used FSB. 
Now you can go to step 2. OR if you don't have a previously installed FSB or just don't want to use this method, go to option 2 below.

  Step 1, Option 2: Determine Hole Location

Before you use this option, you'll need to drill and tap the hole between the sling swivel ears to provide a temporary hold for the FSB. This actually is not a bad idea even if you are not using option 2 as it will help hold the FSB in place during the whole process and it will not interfere with the drill/pin process. Drill the hole between the sling swivel ears using a #21 drill and tap it to #10-32.

 Install a #10-32 X 1/8" set screw, install barrel into receiver and skip ahead to steps 2, 3, and 4 to line up the FSB but use the set screw to hold the FSB instead of the clamp and then come back to this step. See you in minute.......now that your back-next, find the center of the flat areas of the FSB. Using a caliper measure each pad and split the difference both side to side and up and down. Finding the center of each flat area will put your pins in the proper location. More or less. Why did I say that? Because you are performing a semi-precise machining operation without the proper measuring devices or machining equipment.
A side note about pin location: I have not consulted a professional concerning pin placement.
BUT- Nearly every barrel I've seen that previously had an A2 FSB on it reveals that the pins are located directly in the middle (up and down and side to side) of the flat areas of the FSB. This ends up putting more than half of the pin bore is in the barrel, it is not a 50/50 split. I don't think this is a problem with a barrel that has a gas block area diameter of .750". If you truly wanted a 50/50 split of half of the pin in the barrel and half in the FSB, you will need to drop the pins a bit off-center closer to the lower edge of the flat areas on the FSB.

 Your scribed lines should look something like this. Now center punch and/or center drill the locations, re-check your FSB to make sure it is not canted and tight against the handguard cap and skip to step 4 to make sure the FSB has not moved before you start the drilling operation.

Step 2: Line up the FSB with"Table-top Method"

  If you have not done it already from Step 1, Option 2, install the barrel on to your flat top upper and snug the nut. Next, install the handguard cap and FSB on the barrel. Use a small "C" clamp or vise grips with some type of padding for the jaws (you can use a shop rag and some duct tape or a piece of leather works good too) to pinch the front journal of the FSB. (I prefer a "C" clamp over vise grips because you can tighten and loosen it on demand whereas vise grips have to be almost totally released to increase or decrease clamp load.) Snug the clamp enough to keep the FSB from moving but do not crank it down tight. Give it a few light taps on the bayonet lug with a small hammer to push it against the handguard cap. Since the operator end of a flat top has a rounded surface that won't sit flat on the surface, you'll need to come up with a spacer that will square the top rail to the surface. I use a small piece of 1" X 1" square tubing. Find a nice flat surface and put a bit of pressure on the on the barrel near the middle of hanguard area. If you push too hard it will try to rock side to side. Now eyeball it. Does it look straight? If not, try again. If so- go to step 3.

Step 3 Cross-check FSB Alignment with "Level Method"

Using barrel blocks or poly vise jaws, clamp your barrel in a vise and level the receiver. Now move your level under the FSB on the flat machined area of the bayonet lug. It is level? Tweak it until it is and then cross check by putting your level back on the receiver. 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. Look good? Go to step 4.

Step 4: Final check of FSB Alignment with "Eyeball Method"

There's an old saying that a dog will trust it's nose over it's eyes. Well it's the other way around for us humans. Doing your best to keep the receiver level (flat top clocked at noon) sight down the assembly and see if it is straight. It can be deceiving depending on lighting, background, etc..
Go out in the daylight if you have to. You could even mount it on a complete lower and shoulder it to see what you think. The bottom line is- no matter what your table top and your level told you, it has to look straight. If it's straight, tighten the clamp down and spy it again. If it's still straight you are almost ready to start drilling.

Step 5 (optional): Push FSB Against Handgaurd Cap

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. But I did have one failure and the HG cap was not as tight as I wanted when I was done. 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.
Take a piece of aluminum tubing (I chose aluminum because it is easy to work with and won't mar your barrel finish) with an ID of just over 3/4". I used McMaster Carr part number 89965K384.
Cut a piece just over 3/4" long and notch it to clear the bayonet lug. This is the shortest spacer you will ever need as it will work on a 7.5" barrel.

Place it on the barrel with a 1/2" flat washer and your muzzle device. Snug it a bit. You may have to loosen the clamp and set screw holding the FSB in place during the alignment process. 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. Now re-tighten your clamp and set screw and re-check your FSB alignment. It should be good to go.

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.
The logic behind using the short spacer with the notch is you will not have to re-cut a notch in several different custom spacers. The custom spacers used with the short notched spacer will only require straight cuts on the ends.

Step 6: Place Barrel Assembly in Drilling Jig

For this process, I knew I would need something to hold the barrel assembly straight on two planes. 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. So I took a 24" long  piece of 6" channel iron and welded an 8" long piece of 2" X 2" angle iron to it. It is stupid simple and only cost a few bucks to make. Lay your level on the jig then lay it on the barrel. Clamp the flat top to the angle iron and you are good to go. If you don't want to go to the trouble of making a jig, you might be able to use a barrel bench block. (a couple are pictured below)  I've never owned one of these so I'm not sure if it will work. I'm sure there are several other ways to do it. You could move your poly or aluminum vise jaws to your drill press vise and just level it on two planes. Options are endless and only limited by imagination.

                                        

"R Guns"

 Wheeler Engineering

Brownells

Step 7: Time to Drill

Note: Before you drill- if you installed the a set screw in Step One, Option 2, remove it now. It can be reinstalled later after the process is done.
Drilling Process: Don't rush this one. Using a #31 drill bit, start slowly so the bit does not walk and use light even pressure all the way through. Use some cutting lube and take it slow. Let the bit do the work. Do not attempt to drill the other hole until the first hole is tapered and a pin is installed. This is always done "one at a time". The idea is that that the first tapered pin will hold the FSB securely while the other hole is drilled. This will eliminate any chance of the FSB moving during the process.

NOTE: If your barrel has a surface treatment like nitride or melonite, this could be a tough process. I've only done phosphate coated barrels so I can't speak from experience, only what I've read. 

Step 8: Taper the First Hole

Using a tap wrench or similar tool, start reaming the hole BY HAND! This process is best done with with no clock ticking. If you go too deep, it's too late. Ream, fit the pin, ream, fit the pin, ream, fit the pin, and repeat. You'll want to stop reaming before the pin "drops in" to the final depth since it will go a bit farther when driving it home with a punch and hammer. I didn't do any measuring of this, but I'm guessing that the pin will go in about an 1/8" farther from it's "drop in" depth to it's final depth when driven home with a punch and hammer. So be very careful on this step. The goal is to have equal parts of the pin sticking out each side when driven home.

Step 9: Repeat Steps 7 and 8

Level your barrel to your drilling jig (I used a small oak block under the barrel to space it up slightly).
Drill and pin the second location.

Now you're done. That wasn't so bad, was it? I must admit, it was a bit nerve racking on the first one I did. Just take your time and everything will go fine. If you drilled and tapped the hole between the sling lugs to temporarily hold the FSB, you can re-install the set screw if you like.

'Just to make sure it wasn't beginner's luck- I did another one on a 10 1/2" barrel. I am very happy with the results

Backstory:

                                                               

When attempting to gather information/specs on how to install an A2 FSB- I couldn't find a lot of detailed information. I found several videos which were of some help, but they all involved the use of a milling machine, which I do not possess. This one was the most help.

Although an A2 base is not used in the next video, and it's done on a mill with a specialized jig, the drill/pin concept is covered very well and worth a look.

 

 This poor guy tries to do it free hand and obviously does not understand a few basic concepts of the install process.

After watching the videos to educate myself-
 It seemed to me that there are three major obstacles to overcome- 1) Getting the FSB straight on the barrel so it did not end up "canted" 2) Proper placement of the tapered holes 3) How to hold the assembly properly so the holes could be drilled perpendicular to the barrel.

A couple of side notes- 1) Professional machinists or gunsmiths might possibly laugh off the methods I used here but they are missing the point if they do. I'm not doing this for hire and I never will. I did this because I like to tinker and I like a challenge. I enjoy working through problems and coming up with solutions to those problems. The project provided an opportunity to go through a problem/solution process and it will save me around $100 every time I do it.
2) This method relies on the fact that you'll be using the same upper receiver to complete the build as you use in the drilling/pinning process. It is understood that if the upper you use for the process is slightly out of spec., that the FSB might end up canted a bit if you install it on a different upper. The biggest factor here is the location and width of the slot for the barrel extension index pin. If these specs are off, you will likely have other problems- like the bolt engaging the lugs in the barrel extension  improperly, mis-aligned feed ramps, etc... The tighter the tolerances on the upper receiver used for the drill/pin process, the better the chance it will not be canted when installed on a different upper receiver. NOTE: There is a jig available that is designed to line up the FSB to the barrel extension pin. It's a bit pricey but not if you plan on doing a bunch of these. It's available from BRD Engineering.

                                                          (Image from BRD's website)

3) I don't really have any good solid methods for adapting a carry handle upper receiver into this process. I have a few ideas but I don't know how effective they would be. If you are a "frequent builder" and have an extra flat top receiver laying around, you could use it to line up the FSB, clamp it, then move it to your carry handle upper and see if it is straight. Then move it back to the flat top for the drilling/pinning process. Since the time I have prepared this post, I did a third FSB drill/pin install for an upper that has a carry handle. I switched the barrel to a flat top for the drill/pin process and back to the carry handle receiver and everything turned out fine.

Resources

Front sight base components and tooling tooling is offered by many vendors. Here are a few for your convenience:

BRD ENGINERRING

BRD is a "one stop shop" for tooling and parts. They offer drill bits, reamers, FSB's, and taper pins- as well as many other components. There prices are competitive and consistent with other suppliers.

MCMASTER-CARR

An excellent resource for bits, reamers, and tools. Known for their wide selection, high quality standards, and fast shipping.

HARBOR FREIGHT TOOLS 

Known for their low prices and wide selection of  "homeowner" grade tooling.

You get what you pay for but sometimes that is enough. Link above guides you to their smallest "bench-top" drill press that would be the minimum needed to complete an FSB job.

Lastly, I'll just pound out a few links to various vendors/manufactures that carry FSB's and pins.

DAMAGE INDUSTRIES 

BCM

MIDWAY USA

DEL-TON

DPMS

CMMG

Last but not least, Fulton Armory- which has a wide selection of FSB's

FULTON ARMORY