You Have The Tools, Now Here’s How.
After we got the tools in order last time, using them to install a barrel is actually pretty easy.
An AR-15 barrel assembly is a slip-fit into the upper receiver. The barrel extension fits into the opening in the upper and there’s a pin that indexes it in place. A barrel nut then gets turned onto the threaded area that surrounds the opening in the upper receiver and this nut is tightened to secure the works. The nut bears against a flange or shoulder on the barrel extension. The barrel nut has 20 dished cutouts or holes that align with the gas tube hole in the upper receiver. Here’s the thing: one cutout has to align exactly. The smallest amount of contact between the gas tube and the barrel nut can diminish accuracy. By the way, barrel nuts are all essentially the same, whether standard configuration or aftermarket.
Any and all ready-to-go barrels I’ve seen or heard of will unpack with the barrel extension in place. If it wasn’t there then the barrel would be in no way ready to go. Installing an extension onto a barrel blank is well beyond the scope of this article—it’s all machine-shop ops.
Armorer’s manuals call for an installed barrel-nut torque specification that makes it sound like it’s all easy peasy. It’s not always. Most call for 35 foot-pounds of torque. That is a minimum necessary for retention, and then one of the barrel nut holes has to line up. That’s the trick.
The torque specification, then, is really a minimum because it’s likely (highly likely) to take more than that to get the job done. The best thing that can happen, aside from perfection, is having the hole be just a little off such that a little more tight gets it perfect. The worst thing is when it’s sitting about halfway on the other side of aligned and will take dang near another whole hole’s worth of rotation to align with the next available cutout opening. If that’s what it is, then that is also what has to get done. It will take some force. So don’t put the barrel on at less than 35 foot-pounds of torque, and also don’t be surprised if it takes 60 or better to get alignment. It takes what it takes. Now, there is an easy way to make it work—remove metal from the cutout on the barrel nut to attain clearance. I think if there is an excessive amount of torque needed to get alignment, that’s a sane way to proceed, as long as no more than a scant amount is impeding. Use a high-speed hand grinder, like a Dremel tool.
To gauge gas-tube clearance, I use a gas tube alignment tool, which is really just a short piece of round stock that replicates the gas tube. Insert the tool into your bolt carrier key, insert the carrier fully, and check clearance of the gage around the barrel nut cutout. You can make a tool from an old gas tube, or get a sano version engineered for the task.
Take the nut (above) to tight, loosen, and to tight again two or three times. This helps to “seat” the surfaces. A gas tube alignment tool fits into the bolt carrier key (below), and the carrier is then inserted as normal into the upper. Be very critical of clearance, and do this as many times as it takes to get a perfect gap. The final check comes on gas tube installation. The tube should rattle. This tool is from Brownells.
This isn’t fully necessary, but Glen always glues the extension into the upper for a “precision” rifle, such as a competition arm or varmint gun. The glue helps take up some of the gaps that result from the slip fit. Glen use Loctite “red.” Be warned that it will take a lot of heat to remove this bond.
Use antiseize, like Loctite C5-A, on the receiver threads and barrel nut joint. In case there’s a lot of torque necessary for a particular installation, this is insurance against galling, which means the nut won’t continue to progress, which can lead to cracking the upper.
Barrel installation is really all about alignment of the gas tube. Don’t rest until the gap is perfect. Or cheat and make it perfect. Just as long as it’s perfect, it’s good enough. That’s a joke. But true.
To start the process of installation, fit and unfit the barrel nut at least a couple of times by hand to get the threads chased. Then apply a good antiseize compound to the receiver threads. This is a product engineered to ease the stress on such an operation. It’s not oil, but a copper-based lubricant. It will in no way encourage a barrel to loosen. It’s insurance against galling, which can lead to cracking. And, yes, upper receivers can crack during this operation. Evermore reason to select a quality go-between to secure the upper into a vise, as discussed in the July issue (it’s online at www.gunsmagazine.com/digital-editions).
After applying the antiseize, use a breaker bar or torque wrench on the barrel nut to fasten down the barrel to either tight snugness or the recommended torque. If you’re using a torque wrench, switch to the breaker bar to loosen the barrel. As discussed last time, don’t use a torque wrench to loosen, only to tighten. Torque it down, back it off. Repeat one or two more times. Do this to seat the barrel in the receiver extension with the idea of overriding and hopefully eliminating irregularities in this area.
I use high-strength glue to fix the barrel extension into its sleeve in the upper. Right. That means it’s not going to come back off without heat. It also, however, often improves the fit in this area, which, keep in mind, is a slip fit. Some uppers and extensions will combine to create a gap greater than others. The glue is not at all necessary to have a fully functional AR-15 that probably will shoot just fine, but it’s a little step someone looking for the best results really should take. Degrease all associated surfaces before applying the glue, and don’t do it until after test fitting. You’ll have plenty of time before the glue sets.
If the barrel nut gets over-tightened out of alignment, bring it back where it needed to be without worry. As long as minimum torque was attained prior to, there’s no harm in backing it off a tad, and there’s also going to be no other option. Go carefully though. Ultimately, it’s the gas tube that’s installed in the rifle that needs attention to the quality of its fit. The alignment tool just gets the holes to share centers.
Point is—and this is just it—there has to be perfect alignment. Has to be. Work at it as long as it takes. If you don’t, the rifle is likely not to shoot too well. The gas tube should rattle when the bolt is in battery, that means fully forward, locked up. There should be no displacement of the tube in any direction when the carrier key is engaged and disengaged as the carrier moves forward and backward. Feel, listen, look. I place my index finger on the gas tube and then run the carrier in and out. You can usually feel the tube shift as the carrier key engages and disengages the gas tube. If the gas tube is binding it will displace the carrier in the upper. Even just a little has a big influence. I’m convinced that the root reason for the accuracy of the AR-15 is from its “floating” lock up.
If centers are centered and there is binding in the gas tube, then the tube itself has to be bent. Bending a gas tube isn’t easy. It’s stainless steel and will snap with virtually no warning. Rubbing the area of the tube where you’re bending back and forth over a wooden dowel while applying steady pressure helps result in kink-free displacement. Don’t get greedy! If the gas tube rattles, you are good to go.
Information in this article was adapted from The Competitive AR15: Builders Guide, published by Zediker Publishing. Glen is a card-carrying NRA High Master earned using an AR-15 service rifle. For more information, including many downloads, check www.zedikerpublishing.com.
By Glen Zediker
200 S. Front St., Montezuma, IA 50171