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Accurizing AR-type rifles and carbines, Part 2: with some basic steps and after market parts, you can easily convert an out-of-the-box AR rifle or carbine into a respectable group-shooter.

In Parts 1 and 2 in the last couple of issues, I've covered the installation of an aftermarket trigger, flee-floating the barrel, and installing a new barrel on AR-style rifles and carbines. Other minor steps that are available to the gunsmith involve cleaning up and truing some of the more critical surfaces on these guns. This final installment will deal specifically with lapping the upper receiver, recutting or cleaning up the crown, and lapping the crown of the barrel.

Truing the Upper Receiver

After free-floating the barrel, the next procedure I use to achieve good accuracy is to lap the frontal face of the upper receiver's threaded area where the barrel extension mates to it. It's necessary to remove the barrel to perform this service, so when I'm accurizing an existing rifle, I perform this step in the middle of the barrel flee-floating procedure once the barrel has been removed. Brownells (800/741-0015; sells a hand-drill-mounted stainless steel lapping tool (part# 080-000-182) that makes this procedure a cinch.



Even in rifles that group reasonably well, I've found that lapping/ truing the upper receiver always yields a noticeable accuracy gain. An external, wrap-around-style upper-receiver block is needed to lap the face, and since the lapping tool will extend into the receiver during lapping, the ejection-port cover will need to be removed prior to lapping.

The lapping tool is chucked into a hand drill and coated along its length with lubricating oil. The tool can then be inserted into the upper receiver from the front, up to the relief section. The lapping surface of the tool will be directly opposite the receiver face. Wipe the receiver face and the lapping surface clean of any oil and coat the lapping surface with a fine grit lapping compound such as 600. Take care not to allow any lapping compound to get inside the receiver. The relief groove on the tool will serve as a trap for stray grit, thus preventing the removal of the anodizing on the receiver's inner surface.


Insert the tool fully until the lapping surface contacts the receiver face. Run the hand drill no faster than medium speed, while applying forward pressure on the rear of the drill. Lap the receiver for about 30 seconds, then stop the drill and slowly remove it. Slowly spinning the drill in the opposite direction may help, but take care not to score or scratch the inside of the receiver. Once the tool is out, clean the receiver face and inside of any residue and inspect. Most instructions will tell you to remove 60 to 75 percent of the anodization from the front of the receiver, but I remove 90 percent or more when I do it. This procedure is designed to give the barrel extension a trued surface to index on, with equal contact around the circumference.

It normally takes me several repetitions of the lapping procedure before I get the desired result. In tough cases where a section of anodization won't yield, I'll drop to 320 grit, and that usually does the job. It's very important to lubricate the lapping tool each time before inserting it into the receiver. If it gets dry while spinning, the stainless-steel tool will score the inside of the receiver. (If you see smoke, it's already too late.) While light scoring doesn't ruin the upper receiver, it creates a cosmetic flaw and the surface protection provided by anodization will be compromised. Remember that in normal operation, an AR's steel bolt carrier reciprocates inside the aluminum upper receiver, so surface integrity is important for wear resistance.

Once you're done, clean the complete inside of the upper receiver, the newly lapped face, and the threads. The stainless-steel lapping tool simply needs to be cleaned and a light coat of oil applied for storage.

Fixing Crowns

The uniformity of the muzzle's cut at the point where the projectile leaves the bore is critical for shot-to-shot consistency. A flawed crown impedes the projectile's ability to leave the bore with the same flight attitude every time, leading to reduced accuracy. In a perfect world, we'd just leave this transition from rifling to muzzle face at a perfect 90 degrees. However, we've learned the hard way that such a crown is susceptible to damage from even light contact with anything harder than Balsa wood, so, at a minimum, a slight recess is in order for protection. Additionally, I find that most of the non-match barrels I come across will gain a slight accuracy increase once the crown is cleaned up. About half the time, the factory crown is visibly uneven, jagged, burred, or even slightly damaged.

Don't confuse the following procedure with the cosmetic process of making a damaged muzzle face look pretty again. That normally requires filing and use of various sandpaper grits to remove damage and file marks across the entire muzzle face. This is a simple accuracy clean-up procedure aimed squarely at the rifling and the muzzle-face juncture. When reworking a muzzle, I prefer to cut an 11-degree crown whenever possible. For some deeply recessed crowns, however, it may be necessary to use a steeper angle in order to get into the recess.

Using a 1/2-inch-diameter crown cutter from Brownells (part# 080-5986-500) will yield an 11-degree crown. If either a muzzle brake or flash suppressor is present, removing them with a combination tool first will give you clear access to the muzzle. In such a setup, the muzzle brake or flash suppressor protects the crown so the angle becomes less of an issue. The crown simply needs to be freed of any inconsistencies that could hinder accuracy.

Chances are there will be heavy carbon around the face of the muzzle when a brake is removed. This should be completely removed and the bore should also be cleaned before proceeding. Place the barrel in a padded vise, muzzle up, and insert a clean patch approximately two inches into the bore to catch chips and debris. These cutters are designed to be piloted, so once the appropriate brass pilot is installed, lubricate it and the cutting blades with a good-quality metal-cutting oil.

The cutter is controlled with a thread-on T-handle. Once the pilot is inserted into the bore, you must turn the cutter clockwise only to prevent damaging the cutting flutes and crown. Brownells provides good instructions with the cutter, but I deviate from them slightly. I use one hand to push firmly down on the center of the Thandle while turning the cutter (clockwise) with the other hand, approximately five full revolutions. I make a sixth revolution with less pressure, to keep the chip left at the stop point to a minimum.


After the appropriate revolutions, remove the cutter from the bore and use a brush or rag to clean any chips from the cutting flutes. Likewise, clean around the barrel's crown and inspect. I use a lighted magnifying lens whenever inspecting the crown, which aids in seeing imperfections. At this point, it will usually still be too rough to stop. Re-oil and repeat the process two or three more times (five revolutions each) with decreasing pressure on the T-handle each time.



It's important to decrease pressure with the final few revolutions in order to leave a clean stop point on the muzzle. If you see a distinct line (under magnification) running from the bore to the outer edge of the barrel, you're stopping too abruptly and need to back off the pressure to the point of being slightly more than the tool's own weight for the final turns. Once complete, you should see a clean, bright, uniform transition at the desired angle from rifling to muzzle face.


Finally, remove the patch in the barrel and clean the bore well to remove any residual chips and excess oil. For crowns that are recessed too far for a cutter to reach or when burrs are so heavy that the cutter skips instead of cutting cleanly, it's best to use a lathe and the appropriate crowning tools to fix the problem.

Crown Lapping

After repairing or cleaning up a crown, I seldom go straight to the lapping stage. However, if I can't get the crown as smooth as I want or if I see tool marks from the rifling process or other rough spots, I'll lap it before test firing. If the muzzle passes visual inspection after crowning, I'll fire the rifle first and assess the accuracy. If the barrel still isn't measuring up, lapping or polishing the crown normally gives it a slight nudge in the right direction--normally on the order of a 0.1 minute of angle (MOA) decrease in group size.

The materials needed to lap the crown are lapping compound (220, 320, and 600 grits), a hand drill, and a brass muzzle lap. I use Brownells Power Brass Muzzle Laps (part# 080-784-350 for .27-.35 and part# 080-764-243 for .17-6.5 mm) because I have yet to wear one out. While a large, round-headed brass screw will work as a lap, the brass screw will wear out after minimal use. As with most everything from Brownells, the bore laps come with very good instructions (that I only deviate from slightly).

After cleaning the bore, remove the muzzle brake or flash suppressor as necessary and place the barrel muzzle-up in a padded vise. Once again, push a clean patch approximately two inches down into the bore. Chuck the brass lap into a hand drill. This is one case where a hand drill is better than a drill press or lathe; I'll explain why in a moment. Spread a liberal amount of lapping compound around the tip of the lap. I start with an aggressive grit for most crowns--usually 220--though no harm comes from starting with a finer grit. I've found over time that the process is faster and I get the same result if I start with coarse grits and then step down to finer grits. The soft brass of the lap won't harm the steel barrel, and even with moderate pressure it takes a long time to complete the lap so I feel safe starting with 220 grit.

Hold the drill vertical above the AR's muzzle, and place the lapping tool's coated tip lightly against the crown. Set the drill speed as slow as possible, or use minimal trigger pressure and begin spinning it against the crown. Light pressure from the drill's own weight is all that's required to do the job. As the chuck spins, rotate or wobble the drill around in a wide circle so that the entire rounded head of the lap comes in contact with the crown--not just the very tip. Failure to do this will result in a poor lapping job and a hard-to-remove groove around the brass lap's circumference. The wobbling movement is why a hand drill is needed for this lapping method.

I continue in this manner for 30 seconds, and then stop and clean all surfaces and recoat the lapping tool with the same grit. Then I reverse the drill's direction and repeat the process. After working it in both directions, inspect the crown under magnification for imperfections. Once I see the crown beginning to get uniformly smooth, I step down to 320 grit and repeat the process. It may take two or three cycles at each grit before the desired result is achieved. When finished, the crown should be polished smoothly and uniformly, with no alteration of the cut angle and no signs of burrs or "flow" leftover from the rifling process.

Clean the crown and bore of all lapping compound and the resulting brass sludge that will extend into the bore, and remember to remove the patch and clean the entire bore before firing.


The AR is a very versatile shooting platform, and well-suited for multiple uses. While I've covered several steps to accurize the AR platform in the sequence I usually use, in truth they can be performed in any order, depending on your ability to diagnose contributions to inaccuracy. In many cases, it makes sense to perform these steps one at a time, followed by testing for results before proceeding to the next step.

If I've learned anything since starting down the custom-AR road, it's that selecting the proper components, paying close attention to detail in fitting and assembling, and applying careful accurizing procedures all serve to yield a very good-shooting semiautomatic rifle. With a good-quality barrel and ammunition, you can achieve an accuracy of 1/2 MOA or better--without the need for bedding blocks, glass bedding compounds, or replacement stocks.
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Title Annotation:Special Report; assault rifle
Author:Adelmann, Steve
Publication:American Gunsmith
Geographic Code:1USA
Date:Jan 1, 2010
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