Like many of us, I bought something at a gun show that wasn't exactly what I wanted, thinking I could make it work and use the other parts elsewhere. Problem was, I wanted a 16" AR-15 upper with the GI style handguard and front sight. I wanted a rifle to carry in the car or truck that wasn't dependent on batteries to aim. It was a small gun show and the only 16" barrel had a free float handguard. "No problem," I thought. I can use that handguard on another rifle I'm going to assemble. AR-15 building "ain't building" IMO. Just assembling. In the mean time I put a Picatinny rail front sight on it and it shot very low. The sight was intended to mount on a Picatinny gas block so aiming at the 100 yard berm I was shooting into the 25 yard berm. No adjusting that out.
Jumping to the moral of the story, buy a barrel with the sight base you want already installed. It's a lot of work if you're not tooled up to do it regularly.
I scrounged some parts and realized pretty quickly that the previously installed sight was not going to be easy to install. A new sight that had never been drilled would be the way to go so I found a new, never installed F base sight. The upper is an A3 with GI carry handle attached so that was the correct height front sight. It also gave me the opportunity to make the rifle a little lighter like the utility rifle I wanted by getting the free floating railed handguard off. I went with a C.A.R. handguard for its weight and handling as well. It could be built lighter at additional cost but it's OK for what I wanted.
OK, with all of the parts in hand I needed to figure out how to install the sight straight on the barrel. After a lot of searching I found a pair of videos that showed how one guy did it. I followed his instructions and was successful. A lathe and mill are required although it would be possible to do a workaround to get by without a lathe. A collection of blocks to level the barrel on a workbench and a scribe that could be set to the correct height would work. The video shows why a ruler won't work to make the alignment marks. A drill press with a substantial 2 axis vise could be substituted for the mill if it is heavy duty hardware. I would be afraid that light duty tools might slip and ruin the parts and the job.
Part 1: https://www.youtube.com/watch?v=O6BlViDghKY
Part 2: https://www.youtube.com/watch?v=sPf-eZf7wXE
I followed his process with one exception. He had an expanding mandrel to make the required spacer. I had to make a tool to chuck up in the lathe to hold the spacer while I cut the OD down.
The process is to make a spacer the same thickness as the handguard retainer, .025" thick with an ID the same as the front sight boss (.750") and an OD the same size as or slightly larger than the barrel just behind the front sight, .855". That lets you see the yet to be made alignment marks on the barrel and sight and to space the sight so the retainer will fit and the gas port aligns.
The barrel was placed in the lathe and the gas port aligned with a cutting tool and a scratch mark made next to the gas port and one on the barrel's larger diameter just behind the sight area. A corresponding mark is made indexing off of the gas tube hole on the back of the sight. This is the key to a successful installation.
If the barrel nut is not already on the barrel this is a good time to put it on. The delta ring, weld spring and snap ring can be installed later. It can be put on later but must be put on prior to the final installation of the sight base.
The spacer is installed on the barrel followed by the front sight base. The marks are aligned and the sight is clamped on the rear ring with padded Vise Grips. The barrel is placed in the mill vise with a spacer under it so it will remain parallel to the mill bed. With the mill vise already installed with no/minimum runout side to side and the spacer under the barrel 2 of the 3 axes are aligned. A #30 bit is placed in the sling mount hole and the barrel is rotated to be parallel to the bit in the chuck. The final tightening of the mill vise is done.
The point of the bit should align with the bottom of the barrel so that half of the diameter goes through the barrel and half through the sight base. This can be checked with a small straight edge. I used a pointed alignment tool in the drill chuck and aligned it to the mill vise jaw that was against the bottom of the barrel to achieve the same results.
Even though my bits were new I chose to spot drill with a center drill before using a twist drill. That assured no flexing when the hole was started. After spotting the first hole the center drill was exchanged for a #30 twist drill and the hole completed. The bit was withdrawn frequently and cleaned and more cutting oil applied in the hole each time. The pressure was kept light to avoid pushing the sight base out of line. After the first hole was complete a spare #30 bit was inserted in the hole, shank first, with tape around it to keep it from falling through. The Vise Grips were moved to the front sight base ring, the alignment marks checked and the second hole drilled same as the first.
I changed the order of the operations slightly from the videos to satisfy my comfort level and to be able to do one more alignment check before reaming.
At this point the barrel is removed from the mill vise and moved to another vise. Keeping the bit shank in the hole and the Vise Grips on for alignment, verify the alignment marks one more time. The rear hole is reamed from right to left with a 2/0 taper reamer. It is worth the additional expense to not purchase the cheapest taper reamer and drill bits you can find. I bought a $10 reamer and was wishing I had spent more before the first hole was reamed.
As the taper reamer gets 2/3 of the way through it's time to frequently remove the reamer, clean the hole and tapered pin and test fit. When finger pressure gets the pin in with about 3/16" protruding on the big end it's time to tap it in with a hammer. When the small end (left side) is flush or protrudes very slightly when driven in firmly you're done. Tap the tapered pin back out, remove the sight and spacer and reassemble with the handguard retainer in place. Replace the taper pin, ream the other hole as the first one and fit the taper pin to the correct depth.
Remove the sight base and handguard retainer, install the barrel nut, delta ring, weld spring and snap ring if they are not already in place. Replace the handguard retainer and sight base but do not pin.
Install the barrel onto the upper receiver. Apply a high temp grease or aluminum based anti-seize to the threads. Torque the barrel nut to the minimum torque, back off and re-tighten 2 more times. Tightening further as needed, align the barrel nut, weld spring and snap ring so the gas tube will go in place. Slide the gas tube in place, slide the sight base onto the gas tube and finger press the taper pins in place. Install the gas tube roll pin and using an oversized punch, drive it flush to the sight base . Again using an oversized punch, drive the taper pins to their final depth. The hand guards may be installed.
I'll take pictures and add them to the post as soon as I can.
David
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Originally Posted by David2011
