Like any machine, your AR15 will never be 100% perfectly reliable.
Parts will break, magazines will fail, and tight tolerances will loosen with wear. Even with ruthless maintenance, one unlucky clot of dust can shut down your rifle in an instant. With the right knowledge, you can fix most issues without spending money, but the wrong diagnosis can end up feeling like a wild goose chase.
If you’re new to fixing ARs, that’s alright. This blog details most common malfunctions, their origins, and a few solutions, so you can get started on a fix right away.
If you know what you’re looking for, skip to the section below…
- Bolt Won’t Lock
- Bolt Won’t Unlock
- Double Feeds
- Failure to Chamber
- Failure to Eject
- Failure to Fire
- Trigger Malfunctions
- Safety Selector Failure
- Optic Won’t Zero
No matter the malfunction, your first step is eliminating outside variables.
As always, when you’re handling a firearm, the first step is to clear the chamber. If your bolt carrier group is unwilling to lock back, you may have a brass-over-bolt malfunction (also known as bolt over-ride). With the magazine removed, pull the carrier back as far as possible with your charging handle, reach into the ejection port with a finger to hold the bolt back, and slap the charging handle back into place. This method should clear your jam if done correctly. If the jam persists, refer to the ‘bolt won’t unlock’ section further below.
With your rifle now clear, inspect the chamber. A common complaint of ARs is their proclivity for carbon fouling. After a few hundred rounds, the inside of your gun might look like an ash pile. With enough fouling, your rifle will not function reliably, so be sure to clean your rifle fully—including a disassembly and scrubbing for the bolt assembly. Here, high-quality cleaning supplies pay out in spades.
Before reassembly check other key components. You shouldn’t break it all apart yet, but visually inspect the trigger group, gas block, and barrel. If something is busted, bent, or out-of-place, you have your first suspect.
With your rifle now reassembled, inspect your magazines. Magazines are one of the most common sources of malfunction in any AR15. If you’re experiencing malfunctions with brand new magazines, or if you’re using older magazines, the solution could be as easy as changing them out.
Speaking of magazines, when you’re ready to test your rifle’s reliability, bring a few different brands of ammunition. Your rifle might work well with one brand but fail with another, and any additional information can help derive a precise diagnosis.
If your rifle still doesn’t work after these initial fixes, refer to the details below.
Malfunction Diagnosis and Repair
This is our #1 issue—not because it is the most catastrophic, but because it’s the least understood.
Many people never realize that their rifle is severely overgassed, and even fewer know of the real danger involved. An overgassed AR15 won’t just kick and eject harder. The added pressure puts serious strain on your components (particularly the buffer assembly and bolt catch), wearing them out much faster than normal. This will bring even more malfunctions, but more importantly, it can even risk your safety. If your bolt is cycling so aggressively that it bounces on lockup, a primer can ignite while the bolt is out of battery, turning your rifle into a miniature pipebomb. We don’t need to tell you how bad that can get.
There are stopgap ‘patches’ that will make your firearm more safe, but none of these will fully address the problem at hand. The most common answer to overgassing is to add an adjustable gas block and increase the buffer/buffer spring weight. While these will reduce the cycling speed of the firearm, it doesn’t solve the problem. This only sets the strain further on the gas block, which can lead to critical failures down the road.
The only solution to an overgassed rifle is changing the barrel. You need to be sure that your barrel has a gas port size that is both in-spec and matches the rifle’s gas and barrel lengths. Every setup will have its own optimal port size, but there are a few general guidelines to getting the right pressure.
- 10.5in barrels with carbine-length gas systems should use a .070 to .073 diameter gas port. Prefer the low end of the range if running suppressed.
- 11.5in barrels with carbine-length gas systems should use a .071 to .073 diameter gas port. Once again, prefer the low end if you’re using a suppressor.
- 14.5in barrels differ based on carbine-length or mid-length gas systems. Carbines use a .0625 diameter, while middies use a .076 diameter. This compensates for the increased dwell time.
- 16in barrels with mid-length gas systems favor a .078 diameter port.
- 18in barrels with rifle-length gas systems will generally use a .0995 diameter—the largest of the bunch.
- 20in barrels with rifle-length gas systems center on a .093 diameter gas port.
If you follow these guidelines, you’ll be on the right track to having a properly-balanced gas flow and superior reliability.
Bolt Won’t Lock
Most of the time, if a bolt doesn’t lock back after the last round, the magazine is to blame. Weak feed lips, a busted follower, or a worn spring can all cause the rifle to slip on the last shot.
If changing magazines didn’t help, inspect your bolt catch. A dirty bolt catch can limit the movement, preventing the bolt from locking back, so give it a good cleaning. If the bolt catch or bolt catch spring has broken, you can get new ones for replacement.
As discussed in the section above, an overgassed rifle can cause the BCG to skip over the catch, but the same problem can happen to an undergassed rifle as well. Locking failures are an early symptom of short-stroking, so pay attention to any changes in the gun’s ability to feed off a magazine. If it stops feeding new rounds, refer to the ‘Short Strokes’ section below.
Bolt Won’t Unlock
This is usually an all-or-nothing issue.
Either your case is stuck in the chamber or you’ve got a damaged bolt.
If it’s just a jam, you can try mortaring. Collapse the stock to its shortest position, point the rifle straight up, and slam the rifle against the ground while pulling back the charging handle. Be very careful in this procedure, as you can damage your rifle or charging handle if done too aggressively. When done correctly, the bolt will unlock, and the deformed case will fall out. For best results, use a penetrating oil like Kroil to seep in around the case.
If that doesn’t clear it, you may have a damaged bolt, which may require some special training to rectify. Call a certified armorer and have them look at it before attempting anything else.
Double Feeds/Feeding Issues
A common stoppage.
99% of the time, it’s the magazine’s fault.
That other 1% is related to overgassed bolts knocking rounds out of the magazine. This may be from an out-of-spec gas port or light buffer. Either way, check out the ‘Overgassed Rifle’ section at the top for more information.
Failure to Chamber
Failures to chamber include malfunctions where a cartridge is stripped off the magazine but fails to enter battery.
The most common cause of a failure to chamber is carbon fouling. Clean out the chamber, focusing on the locking lugs, feed ramps, and gas tube. Once it’s all clean and dandy, give it another shot.
Failure to Eject
This is an ejector issue or a short-stroking issue.
First, check our ejector and ejector spring. If the spring is weak or broken, you will need to replace it. If you haven’t cleaned your bolt in a while, make sure that you don’t have carbon fouling or brass jamming the ejector. In this case, a simple cleaning should do the trick.
If that doesn’t work, it’s probably short-stroking issues. Follow the steps listed under the following section.
Short-stroking is one of the most frustrating malfunctions, as it has a lot of potential causes, many of which require major parts replacement. This malfunction describes a bolt that isn’t reciprocating far enough into the buffer to complete function. It may extract and eject spent cases, but it will not strip another round off the magazine, turning your AR15 into a single-shot rifle.
The first step is cleaning. The most common cause is just excessive friction from carbon grime. Scrub out the chamber and buffer, including the gas key and gas port. Lubricate your buffer too, ensuring that it slides back with ease.
If that doesn’t work, you’ve probably got a buffer weight or gas issue. Buffers are easy to replace and come in a variety of weights, which are tuned to different barrel lengths and backpressures. A heavy buffer will work great in a short barrel or suppressed AR, where gas pressures are high, but they can cause problems in a low-pressure system. Short stroking is common when the buffer weight is too high, so if you’ve got a heavy buffer installed, try moving to a lower weight. We recommend trying a friend’s buffer before you buy another one, as it can help you determine an ideal buffer weight. You might also try a new buffer spring as well, in case yours has worn out.
If a buffer/buffer spring doesn’t help, you might have an undergassing issue—possibly a leak. If you can borrow a spare bolt carrier group, you can test to see if your gas key is loose or leaking. Also check the gas rings and gap space for proper seal. Otherwise, examine your gas tube and gas block as your gas block might have shifted, or a roll pin might have fallen out. Fortunately, most BCG parts are relatively inexpensive.
Sometimes, carbon will block up the gas tube. In this case, a pipe cleaner will knock all the crud out with ease.
After all that, if it’s still short-stroking, take it to a certified armorer for closer inspection. Something is likely out-of-spec.
Failure to Fire
If you’re getting light strikes (or no strikes at all), you may not have a rifle issue at all.
Many of these malfunctions are caused by your ammunition. Poor quality ammo will have hard primers that refuse to ignite. If you’re using reloads or steel-cased ammunition, this can be a very common issue. Try swapping your ammunition for a different load—or trying the same ammunition in a different rifle.
If malfunctions continue with good ammo, inspect your firing pin. The small tip of the firing pin can sometimes break off, making it difficult to strike a primer. Disassemble your bolt carrier group, inspect the firing pin, clean the firing pin channel, and reassemble it. Be sure you’re not missing any parts, such as the retaining pin, which can sometimes vanish for no reason.
If all else fails, look to your trigger. A weak hammer spring could appear correct, but the reduced speed will translate to weak strikes. In this case, you should only need to replace the spring.
When you pull the trigger, and nothing happens (or you get a double fire), you probably have an issue with the trigger assembly.
Trigger failures are common, since the trigger group has a lot of small parts and springs susceptible to breakage. Disassemble your trigger group and inspect the disconnector, hammer, trigger, and all the pins and springs. If there is any erosion or snapped pieces, you’ve found your culprit.
Be sure to inspect the disconnector hook, hammer notch, and trigger nose as well. These areas are the most susceptible to wear.
Safety Selector Failure
Every so often, a safety selector will lock up or fail altogether.
This often occurs from ingress of sand, carbon, or dirt. Most of the time, all you need to do is disassemble the safety selector and give it a good cleaning. If the selector is broken, you may have to replace it.
In rare occasions, it’s an issue with the trigger, which isn’t engaging the safety due to wear. These parts will need to be replaced.
Rifle Optic Cannot Be Zeroed
Not usually a rifle malfunction, but your rifle optic is a major accessory, so we’re including it just in-case.
95% of the time, the problem stems from the optic’s mount. If your mount isn’t torqued to your rifle’s top rail, you’ll never achieve a consistent zero, as the mount will make very subtle movements with every shot. Refer to your mount’s manual and confirm that you have attached it to the rifle with the correct torque values. If you have a QD mount, check the mechanism for any broken springs or stripped screws, as these can sometimes fail because of recoil impulse.
The other 5% of issues come from the optic’s internals or a slanted top rail. In most cases, the optics manufacturer will inspect your scope for defects, but some companies go a step beyond. Most Primary Arms optics have a lifetime warranty and full commitment to customer satisfaction with a customer service team that always responds within a day.
For Additional Support…
If we’ve helped you with your malfunction, or if you learned something new, we’re happy to help. Malfunctions are frustrating, but there’s certain catharsis when you can fix a rifle to full reliability.
In the case you’re still seeing malfunctions, reach out to us on primaryarms.com. Our customer care specialists can help you figure out your best next steps and advise on any part replacements.