Building the Parts Bin AR Part 1: Reliability

Building the Parts Bin AR Part 1: Reliability

I have an almost AR15 in the parts bin. I’m sure many readers do too. After a few years of building and upgrading AR15s you will likely have a bin full of parts that you might consider sub par or outdated. When you realize that you almost have a “bonus” gun in the parts bin… then it may signal the time to build up a new rifle. I have a spare parts rifle sitting in the box right now. It needs a buffer tube assembly, a barrel, a bolt carrier, hand-guards, and a barrel. The rest of the parts are all sitting in the bottom of my parts bin.

My number one goal in any AR15 I own is to ensure the rifle is reliable.

Ensuring AR15 Reliability:

Magazines and Ammunition

One of the biggest weaknesses of the AR15 lies in the magazine. Mark your magazines with numbers to identify which magazines give you frequent reliability issues. Once identified to be unreliable, throw them away. Old magazines may have feed-lips that have spread too much, or a weak spring that cannot push the rounds up fast enough for the bolt to push the rounds up the feed-ramps. Avoid gun-show magazines unless they are brand spanking new. Aluminum GI magazines with a Magpul follower new in package are DIRT CHEAP.

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Stick with brands like NHMTG or OKAY industries. Go for brand new in wrap Magpul magazines. Go for quality LANCER magazines. There are a few other magazines that are reliable, and a few that are absolute crap. Do your research.

The ammo you stuff into the magazine makes a big difference as well. Take a look at this video from… it is very telling.

Learn more at

Summary? Shoot good ammo, buy good mags, and your overall reliability will improve.

Bolt Assembly and Buffer

The bolt assembly is the next component needed to ensure reliability.  All things considered, the bolt carrier and bolt are the primary players in making sure your rifle goes bang every time you pull the trigger. It is important to buy a bolt assembly that has been tested with the proper quality measures in place. High pressure tested, shot peened, and magnetic particle inspection are the main points to look for to make sure you’re getting a bolt that passes quality control from the factory.

Common issues with the bolt stem from poor gas seal to the gas rings, or a gas key that has started to come loose and leak. Ensure that the bolts gas rings have a good seal by pulling your bolt out, pulling the bolt forward to a unlocked position, and place it bolt down on a flat surface. If the bolt stands up you have a good gas ring seal. If the bolt carrier locks / cams down over the bolt, replace the gas rings. If the replacement gas rings are still not tight, your bolt carrier is overbored and needs to be ditched.

If your gas key is loose, it will wobble under your fingers. Tighten the bolts down and ensure it is staked in place. Any gas leaks are likely to cause failures to feed due to the bolt being unable to completely move to the rear of the buffer tube. Gas leaks cause a loss of energy to the bolt carrier group. Without that gas to push the carrier rear-ward the gun will choke and short-stroke.

Barrel and Gas Port Size

The barrel and gas port is the final piece of the pie. If you are building up a parts bin rifle, you don’t always want to spend $400 dollars on a Noveske barrel. The most important specifications to me are the presence of a properly sized gas port, and a quality 5.56 or “Wylde” chrome lined chamber and bore.


Gas port size and placement on a barrel will help or hinder reliability.

The quality of the barrel steel is a selling point as well… but comparing 4140 steel vs 4150 steel is really a moot point when you are building a beater. I appreciate a longer barrel life of a 4150 barrel as much as anyone, but in reality how many of us have ever burnt up a barrel? At that point you would have spent thousands of dollars on ammunition and the cost of a new barrel would be a drop in the bucket. If you see a 4140 barrel for a great price, snap it up.

The gas port is something most AR owners never think of. Having it properly sized gives the rifle enough gas to operate smoothly with the appropriate bolt carrier velocity. Certain AR15 configurations can increase wear and tear on your rifle, particularly the bolt itself.

What we want is the lowest permissible gas pressure delivered in the appropriate volume to send the bolt carrier fully to the rear.

We do not want an over-gassed gun with a large volume of high pressure gas. This will lead to excessive carrier velocity and bolt wear especially if you are building a over-gassed carbine based gun. A 16 inch barrel with a carbine length gas system will deliver a large volume of high pressure gas.

Furthermore as the gas port erodes from use, the system will be even more over-gassed then when you first started. An over-gassed gun may be reliable, but keep a spare bolt handy to swap it out should stress cracks become visible. 

How to tell? When buying a barrel, it is important to study a manufacturers reputation. Most manufacturers don’t advertise gas port size and thus some research on your part will be required. If the manufacturer has a reputation for quality work then most likely you will receive what you pay for… a quality mil-spec barrel. When budget barrel shopping… do your research.

Wrapping Up:

So when building a parts gun, ensure that the bolt carrier group, barrel, and magazines are of good quality. Each of the above systems depend on the other to ensure reliability. Having things out of balance can increase wear and tear, or in general decrease reliability and weapon durability. If you have something wrong with your AR15 it will likely be related to one of the topics above.

My parts kit build will be receiving good magazines, a quality bolt carrier group, and a chrome lined barrel with the right gas port size and placement. If you have a partial gun sitting in the parts box, take your time and build it slowly and smart. It will be reliable and won’t break the budget.


Written by lothaen


  1. mcthag · March 8, 2014

    Good advice! But it seems to apply more to a first time buyer than someone who’s got a parts bin.

    • lothaen · March 8, 2014

      Yea, if you have a parts bin full of AR15 skeletons then you might know much of the above. Never hurts to review for the new guys.

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