Accurizing AR-Type Rifles and Carbines, Part 2

Last month in Part 1 of this series, I covered the installation of a matchquality aftermarket trigger in the basic AR platform. Once this critical first step is taken, there are still numerous modifications that can be made to AR-style rifles and carbines to help them turn in better performance on the range and in the field. I’ll deal with a few of those here in Part 2. When I’m customizing an existing gun, my next step is to free-float the barrel.

This removes any external and variable pressure from a barrel, and has long been known to help shot-toshot consistency. The design used to attach most AR barrels to the upper receiver lends itself well to this modification. If the stock barrel needs to be replaced, either because of excessive wear or poor performance, this should be done in conjunction with free-floating, since the barrel nut will usually be exchanged for anew one when free-floating. Both barrel replacement and free-floating will be covered here.

When reworking a customer’s gun, I’ll usually free-float the barrel and lap the receiver at the same time. Lapping or "truing" of the upper receiver will be covered next month in Part 3. Free-Floating the Barrel There are essentially two free-float attachment methods. The most popular method is replacing the handguards with a tube that threads or clamps onto anew barrel nut and completely encircles the barrel.

A less common but sometimes easier method involves replacement of the handguards with a two-piece free-float tube that clamps onto the original barrel nut. The latter method is fairly straightforward so it won’t be covered here. After ensuring the rifle is unloaded and cleared, begin the process by driving out the front and rear takedown pins to their captive positions and separating the upper and lower receivers. Remove the bolt-carrier group (BCG) and the charging handle.

An upper receiver block is needed for the procedures that follow. I use both external blocks that wrap around and internal blocks that pin into the upper receiver takedown-pin holes. After placing the unit in a vise, if you haven’t already done so, remove the stock hand guards. Before proceeding, use a combination wrench at this point to remove the muzzle brake or flash suppressor. If a common crush washer was used to tension and time the flash suppressor, remove it as well.

Next, drive the gas-tube roll pin out of the front-sight assembly (FSA) to free up the gas tube for removal. If This .308caliber model from Midway USA (part# 137499) is made of Delrin. It’s easy to use and is impervious to most solvents.

This design also allows unimpeded access to the inside of the receiver from front or rear. This is also a good fixture for installation or removal of the gas-tube roll pin. using an internal receiver block, you’ll have to remove the upper receiver from the block to drive the gas tube rearward into the upper receiver for removal. If the rifle has more than a few rounds through it, carbon will cause the gas tube to stick in the FSA, which doubles as the gas block.

Sometimes light tapping with a brass or rubber hammer will free the gas tube. I usually use a small piece of bicycle inner-tube to grip the gas tube and work it free by hand. Be careful not to bend or dent the tube when removing it. Once free, pull it completely out of the rear of the upper. The gas tube’s manufactured bends will make working it through the barrel-nut assembly and receiver difficult, but it will pass through with some patient work.

The barrel nut can now be loosened with a barrel-nut or spanner wrench designed for the task. Once the nut is loose, remove it from the upper receiver’s threads completely and slide it forward on the barrel slightly. The barrel assembly can now be removed from the upper receiver. Because most free-float tubes require barrel-nut replacement, the stock front-sight assembly must be removed from the barrel. In my view, this is the least enjoyable part of retrofitting an AR.

The FSA is held in place either by tapered or straight pins. They’re pressfit into place and are difficult to remove from most barrels. A special cupped-end tapered pin-starter punch is available from Brownells (part #827- 530-320) or Midway USA and will help hold the punch on the convex pin ends. I’ve found the best method to remove most pins is to place the barrel assembly in a barrel block and use a heavy large-headed hammer and a 3/32-inch punch that’s been ground down to a short length.

This short punch is much stronger than along one and is easier to keep upright over the taper pins when driving. It’s important that the barrel not be stressed during this procedure, so support the chamber and muzzle ends equally without placing them directly on a hard surface. I use a knee pad, applying light pressure on the supported barrel extension (chamber end) to hold the barrel in place. The barrel block holds the FSA in place on the muzzle end.

I also wear a glove on whichever hand holds the punch because it tends to take a lot of abuse while loosening the pins. There’s no easy way to remove these pins and it takes practice to do it without ruining the FSA. I’ve had a few pins that loosened on the first hit and were out in a few seconds, and many more that took me as long as 45 minutes to loosen. I’ve also drilled them out and cut the FSA down the centerline and split it away (if I was not going to reuse it).

Driving the pins out is the preferred method and care must be taken to prevent the punch from walking off the tapered pin ends. Once they begin to loosen, they’ll drive downward through the FSA body and the punch will be captive, but until the movement starts, it will be hard to keep the punch centered on the pins. It’s important to determine which end of the tapered pin is the wider end, as you’ll obviously be driving it out from the smaller side.

This can be difficult to determine sometimes, particularly when the ends of the pins are recessed slightly into the FSA body. I measure the pin ends and compare to determine which is wider and I still get it wrong sometimes. Regardless of what you may read about

the direction of these pins, you’ll find them placed in both directions, sometimes even on the same barrel. To drive the pins out, ensure that punch and pin are lined up properly and hit the punch hard while applying downward pressure with the hand holding the punch to keep it from walking. Light tapping or half-hearted I’m-scared-I’m-gonna-hurt-myhand-or-the-rifle hits don’t work. You have to impart a lot of force onto the tapered pin to get it moving.

Once it begins to move and the punch end becomes captive, you can switch to a full-length 1/8-inch punch to drive the pin through the other side. Then move to the next pin. I’ve found that even straight pins are tight enough that they must be driven out the same way. Once the pins are out, coat the 2inch section of barrel directly in front of the FSA with oil to keep it from scratching the barrel as it’s removed.

Use a lightweight nylon or rubber hammer to drive the FSA off the barrel in the direction of the muzzle. The barrel nut and attached delta-ring assembly will now slide off the barrel. Reassembly If a barrel change is part of the plan, do it now. Since most barrels come from the manufacturer with the barrel extension and index pin already installed, I won’t cover that here. The upper receiver should still be vised in a receiver block.

Prior to installing the barrel, it’s a good idea to chase the threads on the upper receiver and the inside of the new barrel nut with a 1.250-18 tap and die. I’ve found the receiver threads don’t generally need any cutting, but the barrel nuts almost always do. Though not critical, this step serves to true up the interface between the barrel nut and upper receiver, with luck increasing the bearing surface between the two.

When you’re ready to install/reinstall the barrel, start by lightly lubricating the outside surface of the barrel extension with high-temperature grease, taking care not to get grease on the front face of the receiver where the barrel extension’s front lip meets it. Place the barrel extension into the receiver with the index pin at the 12 o’clock position in the upper receiver’s index slot. It should slide in with minimal pressure until fully seated.

The process for replacing the stock handguards with a free-float tube begins at this point. Coat the threads on the mounting surface of the upper receiver with a high-temp anti-seize lubricant, which can be purchased from most auto-supply stores. Apply just enough to cover the first two to three threads very lightly, as a little grease goes along way. If the free-float tube uses its own barrel nut, slide it over the barrel carefully to avoid scratching the finish and thread it onto the upper until it’s hand-tight.

And be sure not to cross-thread it. There are several techniques for barrel-nut torquing, but most gunsmiths agree that 30 to 80 foot-pounds is the correct range. I use a longhandled 3/8-inch-drive torque wrench and a combination wrench (part #TL- 02) from DPMS Firearms, LLC to achieve the proper tightness. Once you reach 30 foot-pounds, check to see if one of the gas-tube holes is aligned with the pass-through hole in the upper receiver. It’s important to line the hole up as exactly as possible.

Even though it can be slightly off center and still allow the gas tube to pass through, the portion of the tube that extends into the upper receiver will be askew. The bolt-carrier key slides over this gas-tube section and any angle will cause it to drag on the carrier key and retard movement of the bolt-carrier group. If the nearest hole isn’t lined up at 30 foot-pounds, continue torquing until the hole is lined up or until you 308-caliber barrel nut would not align within the 30-80 foot pound torque range.

Milling out the metal between them solved the problem without affecting function. This end of the barrel nut is covered by the free-float tube, so cosmetics aren’t an issue. reach 80 foot-pounds. If one gas tube position is below 30 foot-pounds and the next one is over 80, one option is to grind out the meat between the holes and create an elongated gastube hole. The ideal way to do this is with a mill but, lacking that, a combination of rotary tool with die-grinder bit and some needle files will get the job done.

Since this position on the barrel nut is masked against the receiver, the slot won’t be visible when assembled. If your free-float tube uses the stock-type barrel nut, the modification only requires removal of the divider tab between the two respective holes. While it may be tempting to try a barrel nut that indexes on a gas-tube hole slightly above 80 footpounds, you’ll most likely find that headspace will be affected and accuracy will suffer. So it’s best to stay in the range between 30 and 80.

Once the barrel nut is properly torqued and the gas-tube hole is timed, you can install the free-float tube according to the manufacturer’s instructions. If it requires the use of a jam nut,

I’ve found that a small amount of thread locker on the jam-nut/barrelnut interface will allow it to remain tight throughout years of use. I use a pair of spirit levels to time any freefloat tube that has a Picatinney rail to the top of the receiver rails, so any forward-mounting sighting device can be zeroed. In order to use the stock FSA/gas block, the free-float tube must be shorter than the distance from the barrel nut to the gas port, unless you intend to mill out a slot for the FSA.

Long free-float tubes have advantages in some types of shooting, and if desired on a shorter carbine or midlength gas system, another option is to replace the FSA with a low-profile gas block that fits under the tube. If that option is chosen, ensure the gas block has clearance all the way around and doesn’t touch the free-float tube at all. The gas block and gas tube maybe installed prior to the free-float tube with this setup.

For all other configurations, once the free-float tube is installed, slide the FSA or gas block onto the barrel (with the gas tube already pinned in place) and into position over the gas port. You can look through the freefloat tube to line the gas tube up with the appropriate hole in the barrel nut and into the upper receiver. If using the stock FSA, line up the taper pin holes on the FSA with the barrel holes/ slots and reinstall the pins.

If using a replacement gas block, ensure the barrel’s gas port lines up with the block’s port correctly. While the upper receiver assembly is in the vise, place a spirit level on a level surface on the upper, such as a Picatinney rail top. If the gas block also has rail, place another level on top of it and align the spirit levels before tightening the gas block. If no rail surface is present on the gas block, I use the bottom flat and hold the spirit level across it carefully by hand to find the proper alignment.

I recommend waiting until the system has been test fired for function before using a thread-locker on any gas-block screws. Some gas blocks are more finicky than others with respect to their position over the gas port, and it may be necessary to make a minor adjustment. The set screws used to hold gas blocks in place are often very small and any excess torque can strip them or cause a driver to break off in the screw. Next, reattach the muzzle brake or flash suppressor, if present.

Most designs use a crush washer which allows you to time the slots in such away as to aid recoil reduction and/or minimize blast at the 6 o’clock position and limit ground disturbance when firing. A high-temp thread-locking compound is recommended here. After assembly is complete and prior to attaching the upper and lower receivers, the headspace should be checked. Gauges from several manufacturers are available and can be purchased from most of the big parts retailers.

When checking headspace on an AR, remove the extractor, ejector plunger, and ejector spring, and reassemble the bolt carrier group. A bolt-disassembly jig is very helpful if you will be doing this regularly. The ejector must be pushed in nearly flush with the bolt face in order to release tension on the roll pin holding it in place. Either a jig or a third hand will help here. Hold the upper receiver vertically, barrel down, and insert the appropriate gauge in the chamber.

Install the charging handle and boltcarrier group normally, and "throw" the BCG forward into battery, letting its weight and momentum do the work. The idea is to ensure nothing causes a false reading on the "no-go" gauge, such as the ejector applying A Word to Our Readers About American Gunsmith’s Mailing-List Policy Like many other publishers, we make portions of our customer list available to carefully screened companies that offer products and services we believe you may enjoy.

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Whenever a barrel is replaced, it’s a good idea to replace the bolt too. The locking lugs on the bolt and barrel extension both wear, so starting with fresh lugs on both sides is a good idea. Premium barrel makers offer headspaced bolt/barrel combinations. If a bolt won’t headspace on an AR barrel, there aren’t any great options for adjustment, aside from trying other bolts (or barrel extensions) until you find one that headspaces.

This is an imperfect system and impractical for the hobbyist, but for a gunsmith with several bolts, it will eventually yield a combo that works. Conclusions/Recommendations The accurization of AR-style rifles and carbines can be accomplished relatively easily though a series of steps, just as with any other firearm. Thus far, I’ve covered several basic procedures you can perform on a rifle when tightening up its on-target performance.

The installation of a goodquality aftermarket match trigger, freefloating of the barrel, and the replacement of the stock barrel with a betterquality unit are incremental steps that will lead to better overall accuracy. Next month, in Part 3. I’ll cover lapping the upper receiver, recutting a crown by hand, and lapping the crown. All of these steps will add incremental improvements in accuracy to what we’ve already achieved in Parts 1 and 2. ■