Monday, July 14, 2008

Guns: Let's Build an AR! - Part 5 (troubleshooting edition)

This week, I'll be talking about a common AR malfunction, as well as its possible causes and solutions:


A short-stroke on an AR-15 occurs when the bolt doesn't go back into the receiver far enough to strip the next round off the top of the mag. There are two main causes of this malfunction: 1) not enough gas pressure in the first place and 2) too much resistance to the bolt carrier's movement.

The gas system of the AR has been much maligned, but there are a few simple things to check. First and foremost, of course, is the carrier key. If that's squared away, check the gas tube, both for wear inside the upper, as well as wear underneath the handguards. If you see that the "mushroom" of the gas tube inside the upper receiver has been damaged, for instance, that's probably causing gas to escape from the tube and weakening the rearward impulse needed to cycle the bolt. Additionally, make sure the gas tube roll pin is installed correctly and that the tube isn't leaking carbon around the front sight base (you may need to take a different AR to compare if you're not sure what is an acceptable amount of leakage looks like). Try dripping some CLP into the tube from the receiver and seeing how it flows into the barrel to check for any blockages. If your gas tube is broken, you can replace it, but it's not a quick process if you don't have the tools.

The second part of the examination looks at the mechanical parts of the system - the bolt carrier group and the components involved in its rearward travel. First of all, check the chamber and locking lugs of the bolt, since a sheared lug, while not normally a cause of this malfunction, is bad news. Make sure the rails are in good working order (no significant cracks or burrs), and that the bolt's gas rings are still holding up. You can test the rings by taking the cam pin out - the bolt should not fall out of the carrier even with the pin removed, and the extended bolt should be able to support the weight of the carrier without the pin even when stood vertically on the bolt face. Finally, make sure the buffer and buffer springs are the correct sizes. Don't take more drastic measures (like clipping coils off the buffer spring) until you can get someone who knows ARs to look at it.

As you might recall, my first experience with my new AR wasn't particularly pleasurable. In order to solve my short-stroking problem, I checked everything I listed above, and the only thing that was out-of-the-ordinary was how hard the bolt was to manually cycle. I figured there was too much friction between the carrier and the rest of the gun, and it was that friction which was preventing the action from cycling completely. So, I took some high-temperature lithium bearing grease, covered the rails with a light film of the stuff, and worked the action by hand for about half an hour (maybe a couple thousand times). By the end of that time, like I expected, the grease had turned into a slurry of debris, and it had smoothed out the cycling of the gun considerably. I cleaned out the excess grease, and headed to the range.

Did all the hard work pay off? Well, the rifle now feeds pretty flawlessly, cycling through 120 rounds without any kind of stoppages. It even locks back consistently on an empty mag, which is a sure sign that the system is in good working order and not short-stroking. Woohoo!

So there you have it. The carbine is put together, and functions correctly; the "build" part is done. Just like owning a guitar doesn't make you a musician, though, owning a rifle doesn't make you a rifleman. Look for more in the coming months about ammo, sights, and all sorts of AR gadgetry.

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