AR-15 gunsmithing for the novice: installing a barrel: part 2: Getting the barrel nut properly aligned with the receiver can be annoying, but Matthews shows you three ways to get it done.
What you have to do is evenly remove a small amount of material from the front of the receiver. This will allow the nut to turn on a bit further and allow the notches to align. The amount of material being removed is very small, less than .003".
The threads on the front of an AR 15 receiver are of 18 per inch. This means that one complete thread revolution moves the nut .055".
There are 20 notches on an AR-15 barrel nut. So moving one notch relative to the receiver means .0027" or roughly three thousandths of an inch.
If, for example, you were off alignment by a half notch, removing a little over .001" (actually 1.35 thousandths) from the front of the receiver would allow the nut to screw on far enough to align the notches with the gas tube hole.
As you can see, it doesn't take much material removal to get things aligned.
An easy way to remove this very small amount of material is to place a piece of fine sandpaper (220- to 400-grit) on top of a flat steel plate.
You then carefully push and pull the front of the receiver back and forth evenly to remove a little off the front.
You can also use a twisting motion to sand material off the front rather than pushing and pulling. The most important thing is to do it squarely. If you don't do it squarely your barrel will be mounted at a slight angle.
Remember to do this carefully; once material is removed, it can't be put back on. It is best to remove minimal amounts and then test fit multiple times to prevent taking off too much.
Once you get the notches to align with the proper torque, your barrel installation is complete except for installing the gas tube.
The next barrel that I installed was my Stag Arms heavy barrel. This was also installed on a flattop upper receiver. In fact, all barrels in this article were installed on the same type and brand of receiver.
Recently I got three Anderson Mfg. A-3 flattop receivers from Copes Distributing. These were cosmetic defect receivers with slight finish blemishes that did not affect function in any way. They were priced at $39.95 and I couldn't pass up the bargain.
This time I used something considerably better than pieces of scrap wood to hold the barrel: the aluminum vise jaws offered by Brownells. These were the type that featured polyurethane inserts bonded to the aluminum blocks.
The polyurethane securely holds the barrels without the chance of damaging the finish. They do have a somewhat spongy feel in use, but I have never had a barrel slip in the jaws when tightened down fully.
I placed the barrel in the vise with the jaws as close to the breech as possible. I only left enough room to allow the barrel nut to be placed over the barrel and allow for use of the barrel wrench.
This time I used the GI-type combination tool and a long breaker bar. Remember to install your delta ring assembly on the nut before installing it and apply grease to the threads. I then placed the pegs of the tool into the notches of the barrel nut.
My GI-type multi-tool looks a little different than most because I did some modifications to it. My tool has an extra peg in it.
On some past job, I abused the tool and sprung the front portion with the pegs and pushed them apart so that they no longer fit well in the nut notches. When I pressed the tool's pegs back in place, I decided to add a reinforcement plate to the front. Since I had the plate added I decided to place an extra peg in the plate to grip the nut better.
Once the nut was turned on hand tight and the tool was securely engaged, I began tightening the nut. This is best done in three stages.
First I tightened the nut to roughly 15 foot-pounds, loosened it, and then tightened it to about 30 foot-pounds. This was followed by loosening again and then final tightening to at least 35 foot-pounds (I actually prefer at least 40.)
This three stage tightening method, according to many AR gunsmithing sources, allows for more accurate torqueing of the nut. Well, as usual, my notches did not align with the gas tube hole in the receiver.
Going to the maximum torque of 65 foot-pounds still left it about 1/3 of a notch short of aligning. This meant I would have to face the front of the receiver just as on the previous job.
To remove less than .0005", I used a wide fine-cut file. Just as with the sandpaper method, you must do this very accurately. You must evenly and squarely remove the material on the front of the receiver. Remember once removed you can't put the metal back on.
If by some chance you take off too much, your only option to save the receiver is to take off enough material to allow the nut to tighten one more additional notch besides the amount needed to originally align the notches.
It is best to do just a little filing, followed by checking several times to prevent overworking the front of the receiver. After a few times filing and checking, I finally had the notches aligned with the gas tube hole when tightened down. Whenever I have to face a receiver I try to take off just enough material so that when aligned I have about 45-50 foot-pounds of torque applied.
It was now time to install my big-buck match-grade barrel. This time, I used the Brownells AR-15 receiver blocks. These hinged blocks surround the front of the receiver so it is fully supported.
The blocks also help prevent marring the finish or smashing the receiver when it is tight in the vise. A long section of plastic rod is supplied with the blocks to act as a support on the inside of the receiver.
I use the rod when tightening the barrel nut, but obviously I have to remove it when I use my bolt carrier to check notch and hole alignment.
When using these blocks, do not tighten them down in the vise till you have a barrel installed in the receiver. If you tighten without a barrel installed, the barrel hole can be deformed enough that you cannot get a barrel into the receiver.
On this job, I would not be using a regular GI style barrel nut. The DPMS-free float handguard (Brownells #231-015-006. $60) features its own unique barrel nut that is part of the handguard.
This barrel nut is a wide, thick collar that has evenly spaced holes in it rather than notches. The holes are spaced the same as the notches, so I could use my DPMS combination tool.
The two pegs in the tool will fit in two holes of the barrel nut. With no delta ring spring trying to push the tool out of engagement, it is easier to install this type of barrel nut.
Just as before, I applied grease to the threads and then used the three stage tightening method. Just as with the previous jobs, 35-65 foot-pounds of torque did not allow for exact alignment of the holes in the barrel nut and the gas tube hole in the receiver. I was off about 1/3 of a hole.
Rather than using the sandpaper or filing methods of working the front of the receiver I used a very handy self-made receiver facing tool. This tool serves two functions. It removes material to allow the nut to turn on further, plus it precisely squares up the front of the receiver.
This tool is easily made if you have a metal lathe. First you turn a 6-inch piece of steel rod down to just a couple thousandths under an inch. This will allow it to slide easily into the AR-15 upper receiver. Face both ends so they are perfectly square.
In the center of one end, drill and tap a hole for a 3/8 bolt. Now take a 2 to 3-inch piece of 1 1/2-inch rod and face both ends square. Drill a 3/8" hole all the way through the rod.
On one end, you need to attach a 1 1/2-inch self-adhesive sanding disc. Grit should be 80-100. Cut out the center of the disc so a bolt can pass.
Use a long 3/8" bolt to connect the two pieces of rod.
Once it's completed, you should have a tool that will precisely face the front of the receiver square. When aluminum builds up in the abrasive disc, use a wire brush or pressurized air to clean it.
Since I made this tool, I never use the sandpaper or filing methods. If you are going to do much AR barrel work it is well worth it to make this tool. It is very inexpensive (only a couple dollars for material), very easy to use, and allows for accurate receiver facing
You can buy a receiver facing tool from Brownells, but this one is so inexpensive and easy to make, I can't recommend you spend big bucks on a factory made tool.
Once I trimmed my receiver up with my self-made tool, I installed my barrel nut to proper torque and alignment, checking with my drill bit in the carrier key.
The barrel nut is part of the DPMS free-float handguard; the front portion screws onto it. Since the handguard is round, it is hard to grip tightly when tightening. The easiest way to grip the handguard is with an inexpensive non-marring rubber strap wrench.
These are available for only a few dollars from places like Grizzly Tools or Harbor Freight.
The strap wrench should allow you to get the handguard plenty tight, but if you are concerned about it coming loose just apply some Loctite 242 thread locker to the threads during assembly.
Be aware, however, that the more solidly you secure the parts together, the harder they will be to take apart later if needed.
If you use thread locker, you will likely have to use a torch to heat the parts and soften the thread locker before you can get them apart. I wanted my handguard very tight on this project so that's the way I did it.
Unfortunately, I later needed to remove the handguard when I decided I wanted it on another rifle. It was a real bitch getting the handguard apart once it was glued tight with thread locker!
Some of the higher strength thread lockers will almost permanently bond the parts together and make removal almost impossible unless you really heat up the parts to break the bond. Take this as a warning from someone who has been there, done that!
I would be using a DoubleStar Picatinny style gas block. This block is secured with set screws. Brownells offers it under part number 100-004-536 and prices it at $32. The installer must align the gas block with the gas port in the barrel.
There is one very easy way to do it without the need to buy expensive tools or fixtures. An easy way to align the gas block is to lay a straight edge (a scale or ruler) on both the top of the flat top receiver and on top of the gas block.
All you have to do is to sight over the two straight edges and align them so they are parallel. When the gas block is even with the top of the receiver, the gas port should align with the gas hole in the gas block. Once the straightedges are aligned, you can tighten the set screws in the block. When tightening the screws, make sure you do not move the parts out of alignment.
Caption: Sometimes even a long wrench won't generate enough torque to get the barrel nut aligned with the gas tube hole. That's when you may need to face the receiver.
Caption: The simplest way to face the upper receiver is to lay fine sandpaper on a flat surface, then carefully and squarely sand a very small amount off the front.
Caption: The receiver can be trimmed by careful use of a wide fine cut file. This must be done squarely. Less than one thousandth of an inch will need to be removed.
Caption: Matthews' shop-built facing tool makes it easy to shorten and square the front of the receiver for proper barrel nut fitting. It takes out the guesswork.
Caption: Round free float handguards are difficult to grip tightly by hand. An inexpensive strap wrench will allow you to grip them and tighten them snugly.
Caption: If you are concerned about the handguard coming loose during use, the application of some Loctite #242 at assembly will help hold the guards in place.
Caption: Removing the last ridge before installing the Double-Star gas block will allow you to install or remove the handguard without having to remove the block.
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|Date:||Jun 1, 2017|
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