AR Family1Add my vote for this tag Gunsmithing1Add my vote for this tag create new tag
view all tags
Share this page
Twitter Delicious Digg Facebook Google Bookmarks Reddit StumbleUpon

Installing a Used Front Sight Base onto a New AR-15 Barrel

This article is a work-in-progress. Check back soon for more information.


Century Arms has been selling a complete Colt M16A1 parts kit with a brand new stainless barrel.

The kit contains everything to build a functioning M16A1 rifle with the exception of a lower receiver and disconnector. Anyways, this kit provides a front sight block that has already been drilled once. Normally the FSB is considered part of the barrel, and if ever damaged in the field the USGI armorer will simply replace the barrel assembly with a new barrel and FSB combo.

If you're building one of these kits up into a retro M16A1, certainly you don't have that luxury. This article details the steps in re-using the FSB on the virgin barrel. The method described here forgoes the use of taper pins and instead uses screws to secure the FSB to the barrel. I know, I know... The purists among us are fuming already. But, this method is easy enough for anyone with a basic knowledge of machining to accomplish, and doesn't require any expensive, specific machine tooling.

If you're at all squeamish, and don't feel like potentially ruining an expensive part of your rifle, contact Randall at AR15barrels.com. For a reasonable fee (I was quoted $65 but check with him for current pricing) he'll install a new FSB on the barrel. He also does custom parkerization work, so if you really want to restore it properly without expending a whole lot of effort, he's the guy to go to.


The proper method to mount the front sight base to the barrel involves using a FSB installation jig. This is a device into which the barrel extension slips, then has a slot that holds the FSB aligned with the barrel extension pin. The sling swivel holes and front sight pin holes are used to keep the barrel aligned with the FSB. The holes are drilled simultaneously into the barrel and FSB with a #31 drill. A 2/0 taper reamer is used to ream the holes to the exact size to allow the pins to fit perfectly when hammered.

This usually means that a front sight base is "married" to a particular barrel. So, when swapping barrels you'd normally install a new front sight base and follow the procedure above to permanently mount it. It seems a shame that an original USGI part would be relegated to the scrap bin, especially in a complete kit such as this. So, we will re-use it. Since it's already been drilled once before, we'll try and keep it as close to the initial dimensions as possible.


Tools/Supplies Needed

  • A #29 HSS or cobalt drill (a #29 carbide cutter would work better, but that violates the "expensive/specific" constraint)
  • A #8-32 tap
  • One #8-32 random screw about 1" in length
  • Four small steel #8-32 setscrews (I forget the length - will find out and update)
  • Multi-speed drill press or vertical mill
  • High-sulphur cutting oil (if you can't find it normal cutting/machining oil will work fine)
  • Two drill press vises
  • Rubber mallet
  • 6"-8" C-clamp
  • Small bar level
  • Cyanoacrylate glue (Super Glue or Krazy Glue)
  • Acetone or nitromethane
  • Thread locker (Loctite #271 - the red one)
  • Cleaning brush
  • Brownell's upper receiver clamshell vise
  • Barrel nut wrench
  • Torque wrench
  • The dreaded T-handle (your shop teacher told you never to use this but we're going to anyways. Trust me)
First, mount the barrel to an upper receiver using the clamshell receiver vise and torque it to spec (30-50 lbs/ft). In order for the installation procedure to be exact, you need to have the barrel installed the way it would be installed in the finished weapon, so make sure that the gas tube is properly indexed at this time.

Next, install the triangle cap for the handguards, then press the front sight base onto the journal. It should be snug, but you should be able to do this by hand. If you think you'd need to beat it on with a mallet, take some #600 sandpaper and take the journal down a couple thousandths. Anyways, you need the triangle cap in place to use as a spacer.

Next, we'll use a trick to align the carry handle rear sight with the front sight base. Remove the upper receiver assembly from the clamshell vise at this point. Find a very hard, flat, level surface, such as a kitchen countertop, table saw table, etc. Flip the receiver assembly upside-down and rest it on the countertop with the sight hoods against the surface. When the front sight base is perfectly aligned, the assembly will rest on four points; two on the carry handle and two on the front sight base. The goal here is to align the front sight base so that all four points are touching the surface. Once this is complete, get out your cyanoacrylate glue and use a sparing amount to affix the front sight base to the barrel. A couple drops will do fine. Allow the glue to cure for fifteen to twenty minutes. Once the glue has cured completely, the front sight base shouldn't rotate without breaking the glue bonds. This will keep it from becoming misaligned when we install it in our makeshift jig.

You will more than likely need something to extend your drill press table. You can do this with something rigid like a piece of 3/4" MDF clamped to the drill press table. Use your imagination. Also, if your drill press isn't properly set up (that is, leveled and adjusted), stop here and do that first. The drill and mandrel should be exactly perpendicular to the table, which should be level. Set the speed to the fastest setting possible (yes, your shop teacher is furious). We need to do this at a fast rotational speed, removing as little material as possible to avoid allowng the drill to deflect, ruining that expensive barrel.

Put the upper receiver back into the clamshell vise, but this time use the C-clamp to hold the clamshell closed. Rotate the assembly so the long axis of the clamshell is horizontal, then install the clamshell into the first drill press vise. Clamp the second drill press vise around the front sight base. It'll be a little cock-eyed, but note that it's really just there to make sure the barrel stays level in respect to the drill press table when you make your first cut. Use your bar level to ensure the barrel is held level and adjust as necessary.

Now you're ready to make the first cut. Put several drops of cutting fluid into the existing hole in the front sight base and on the drill. The #29 drill will almost exactly match the diameter of the larger of the two taper holes, so align the drill with the workpiece such that you're not ovalling the hole. Switch the drill press on, and very, very slowly begin to remove material. The slower you go, the more accurate (and polished) the final cut will be. Clean the cut often and re-add cutting fluid as necessary.

Once you're through to the other side of the front sight base, you can tap the hole. Clean the cut, then apply some fresh cutting fluid. Install the #8-32 tap into the T handle and give it a liberal dose of cutting fluid. Remove the #29 drill from the mandrel and instead chuck up the T handle. No, we're not going to cut the threads with the drill press (you'd give your shop teacher a heart attack). The reason we're doing this is to make sure we get a clean, centered start to the threads. Align the tap with the newly-cut hole and start cutting the threads by manually rotating the drill press mandrel. Once you've started the threads you can loosen the chuck and finsh them by hand. Clean them when you're done and put in a screw to make double sure everything is secure.

Repeat the cut and tap steps for the second hole. Remove the screw securing the front sight base to the barrel, then remove the upper receiver assembly from the makeshift jig, including the clamshell vise.

Now, we can remove the cyanoacrylate glue. A few sharp raps with the rubber mallet to the front sight base will break the weak glue bonds. Get a small aluminum tray and fill it with the acetone (works OK) or nitromethane (works better). Remove the front sight base from the barrel (if possible) and soak it in the solvent, which will slowly dissolve the cyanoacrylate. Dip your cleaning brush into the solvent and brush the barrel until any traces of the glue are gone. Repeat the process for the front sight base.

Once all the glue is removed, press the front sight base back onto the barrel. The reason we have four set screws is so that we can maintain opposing tension, preventing the screws from backing out. Place a drop of thread locker onto the first setscrew and drive it in until the head is flush with the surface of the front sight base. Do the same on the other side of the hole, evening the two screws so that the same amount of screw is either sticking out or recessed within the FSB. Repeat for the second hole.

Check to see if everything is aligned at this point by placing the receiver, sight hoods down, on the flat surface again. If it's sitting on all four points, you've done everything correctly and you can finish assembling your rifle. If it's not, you can correct this with your rubber mallet. A few quick raps on the front sight base should help align it (there's always a little bit of play, allowing the barrel to rotate in the receiver).

At this point, you've installed your front sight base and can continue assembling your rifle. If you haven't yet parkerized your barrel, you should do that now (but make sure it's completely degreased first!).

Edit | Attach | Watch | Print version | History: r3 < r2 < r1 | Backlinks | Raw View | Raw edit | More topic actions
Topic revision: r3 - 15 Dec 2010 - SeanNewton
Would this information help out someone you know? Click here to share it! Twitter Delicious Digg Facebook Google Bookmarks LinkedIn Reddit StumbleUpon

This site is powered by the TWiki collaboration platform Powered by PerlCopyright © 2008-2023 by the contributing authors. All material on this collaboration platform is the property of the contributing authors.
Ideas, requests, problems regarding TWiki? Send feedback