All AR-platform firearms have a receiver extension tube, called a buffer tube. That is where the buffer and operating spring ride. It’s also the part that serves to mount the buttstock shell and allows to enhance or hurt your performance depending on the stock you choose.
Rifle Specs
All rifle-pattern AR-15s use the same extension tube—10.125 inches long, 1.115 inches in diameter. It’s an aluminum piece (7075 or 6061) that threads into the lower, and that’s about that. However, put some adhesive on those threads. Use light-duty Loctite Threadlocker (blue or purple is plenty), and make sure to degrease the threads prior to use (I use brake cleaner). Snug it to 35 ft-lbs, which, without benefit of a torque wrench, means “pretty snug.”
The rifle tube is the same now as it was on the original A1. The longer A2 stock shell needs a 5/8-inch spacer added to the end of this tube, and either one then takes the same (but correspondingly shorter or longer) screw to affix the stock shell.
Collapsible Specs
The CAR-style buffer tube is, first, shorter. This is to allow the collapsible buttstock to, well, collapse. I still call it a CAR because that’s what Colt’s called it when the second-generation collapsible appeared way back on the Colt’s Commando, the CAR-15, which established the dimensions and configuration still used today. That one is a little more involved, and that’s the focus of the rest of this article.
The first thing you’ll encounter when shopping a new stock for your carbine is whether you need a mil-spec or commercial-spec buffer tube. The threads on either will fit the tube into the lower receiver. Mil-spec threads are 1.185 inches compared with 1.170, but the receiver accepts both. However, other dimensions are different, most importantly the tube body diameter.
Mil-spec is a smaller-diameter tube. The tube and the stock shell have to match. Mil-spec body diameter is 1.148 inches, and commercial is 1.168. Measure the tube body, not the threads.
Why? After Colt lost its commercial-production exclusive on the AR-15, there were a lot of clones on the market. Not all were up to spec, literally. That’s where the commercial tube originated. Its manufacturing process is easier and less expensive.
Mil-spec tubes are impact-extruded from 7075 aluminum alloy. Commercial tubes are made from 6061. The higher-strength 7075 can be thinner-walled, or, more correctly, the 6061 has a thicker wall to have the same strength. Interior diameters are the same. Another big difference is that the threads on a mil-spec are rolled on (another extrusion process). Threads on a commercial are cut, like a water pipe, and that’s the reason for the larger body diameter. Keep in mind that there are really no set standards for commercial tubes as there are for mil-spec. It’s most common for a commercial tube to have a slanted end. The tube, though, is flat on the inside; the angle is only cosmetic. Mil-spec has a flat end cap that’s integral to the tube body.
There’s no glaring performance difference comparing commercial with mil-spec, but the threaded portion on a mil-spec tube is factually stronger. It’s unlikely that will ever matter, but this does matter: The main advantage of a mil-spec is that some of the better stock designs are available only in mil-spec. That means more, better choices. I don’t think there’s any other reason to replace an existing commercial with a mil-spec, but I decidedly will recommend starting off fresh with a mil-spec.
Last
Beware, there are now tubes that look like mil-spec but are manufactured using the less expensive commercial-style means and materials. The giveaway is the thread diameter. Recollect that it’s a little larger—visibly larger—on a true mil-spec. Next is that there are pistol buffer tubes. Some of these are proprietary to various stabilizing brace manufacturers, but, otherwise, a mil-spec buffer tube works fine. Always check before you buy, and it’s usually best to get the complete package.
Which one? There are many good ones, about a gazillion in all. They vary in details and features such as battery storage and sling attachment options (some even include detachable mini-magazines, nice touch). Price is a leading indicator of quality, true enough. Look for something that’s actually employed (deployed) as a mil-contract piece, and you can’t go wrong. Don’t get a cheap stock! It will wiggle and then rattle.
You can see Glen Zediker’s installation instructions in this free PDF download.
Do you have a favorite stock for you AR? What about the buffer tube? Share your answers in the comment section.
The preceding is a specially adapted excerpt from the author’s newest book, “America’s Gun: The Practical AR15.”
Great article. There is definitely confusion out there between what Mil-spec and Commercial tubes and stocks really ARE, and the differences, especially among those newer to the AR-15 platform, and this article nails it! Great job!
The burgeoning variety of calibers and gas-vs-piston actions available for the AR platform has also resulted in expanding choices for buffers and springs. If you’re trying to make one of these odd builds work reliably (that’s where the fun is, IMO), you might consider discussing light vs. strong buffer springs, along with lighter or even split buffers. A 3 oz buffer can be split into two 1.5 oz buffers in the tube, and this approach has really assisted reliability for me.
There is a strength difference between the materials and dimensions of the carbine extension, but I have not seen a definitive breaking test comparison.