“What do I need to be aware of when choosing parts for an AR-15 build?” This is one of the most common questions that I am asked on a weekly basis through my YouTube channel, Facebook or via email. To be honest, I find this to be a much broader and complex subject than some may think. However, that is probably because I am self-diagnosed with the bank-account-depleting-black-rifle-disease and am slightly obsessed with this variant of rifle—slightly.

When venturing into building an AR-15, there is a vast, and sometimes overwhelming, amount of information available both in print and online. Based on my own experiences and knowledge gained, I believe there are some areas and aspects of an AR-15 build that are more important than others.

10.5" AR-15 Pistol - thearmsguide.com
This is my 10.5″ AR-15 build. It is far from “mil-spec” but it is built the way I want it.

To reiterate, the following are my personal opinions and you will almost certainly discover varying opinions during your own research (with so many options, how can everyone agree on all of them?). Here is some information on what I think, what I look for and what I pay attention to when selecting components for my own AR-15 builds.

The Mil-spec Quandary

This one tends to be a bit of a sensitive subject among AR aficionadaos, after all, the AR-15 is “America’s rifle.”

Unless you obtain a select fire M4 or M16 manufactured by either Colt or FNH, your rifle will not be built 100% per military specifications. Sorry, but it’s true. And, you won’t be obtaining one unless you have a FFL07/SOT or live in a free state where you can find a transferable rifle while also having a small fortune to spend. As a civilian, purchasing a Colt 6920 AR-15 is about as close as you will get to owning a definitive mil-spec rifle. However, this is not to say that I don’t look for and/or prefer components labeled as “mil-spec.” In fact, I do, especially for the sake of compatibility. Allow me to explain.

There are very specific dimensions, tolerances and measurements that AR-15 component manufacturers follow—military specifications if you will. These are important to assure the compatibility, reliability and interchangeability of all AR-15 parts and pieces that are made from various manufacturers. When speaking in terms of “mil-spec,” these are the specifications that matter: the dimensions and tolerances.

Here’s an example of what I mean: my MEGA Megalithic billet upper receiver, MEGA billet lower receiver and FailZero bolt carrier group (BCG) technically do not meet military specifications. Instead of being forged aluminum, my MEGA receivers are billet aluminum. Instead of being parkerized, my FailZero bolt carrier group is finished in nickel boron plating.

A billet receiver and nickel boron plating is not mil-spec. However, all of the components I mentioned have dimensions and tolerances that are per mil-spec. This is what allows me to install the barrel, lower parts kit and use a bolt carrier group all from different manufacturers without having to modify parts to assure successful “pewpewpew.”

Megalithic Upper AR-15 Build - thearmsguide.com
My one piece MEGA Megalithic upper receiver sure isn’t mil-spec, but it is the highest quality upper receiver I have ever owned.

Personally, I recommend that you take the time to use your Google-Fu and research whichever manufacturer whose you are planning on utilizing component. During this research, be wary of overly positive or negative reviews. That will help you weed out some of the more biased viewpoints.

Focus Your Spending

If you have a build budget, which many of us are not at the luxury of avoiding, invest a significant amount of your budget on the barrel and the bolt carrier group. An AR-15’s barrel and bolt carrier group are two of the most important functional components. Without a quality barrel and bolt carrier group, your rifle’s accuracy and reliability will suffer. Especially when working on a home defense build, you do not want a failing rifle when you need it most. After all, bad news never has good timing. Here are some of the features I look for when purchasing barrels and bolt carrier groups.

AR-15 Bolt Cam Out Measurement - thearmsguide.com
This is a picture of me measuring the ejector cam out on my BCM bolt. The brass markings on the bolt face are from the factory when they test fired it. If it passed inspection, then I know it will certainly work for me.
  • Instead of using AR-15 versions, I only use M16 bolt carrier groups. The rifle was originally designed to work with the M16 BCG, so that’s the design I choose to trust.
  • I prefer shot peened bolts made with Carpenter #158 steel that have been high pressure tested and mag-particle inspected. I don’t feel there’s anything wrong with batch testing, but if given my druthers, I prefer knowing that the bolt I purchased was individually tested and inspected. Bravo Company USA is one manufacturer that produces a high quality bolt carrier group.
  • Gas key staking on the carrier is an absolute must-have for me. I want to see material literally manipulated into the gas key screws. This gives me peace of mind knowing that the gas key will not come loose and will serve me with the utmost reliability.
  • For their reliability, I look for M4 feed ramps in my barrel extensions and upper receivers. Nowadays, M4 feed ramps are already a common feature in upper receivers. Colt designed the M4 feed ramps to improve feeding reliability of the rounds in their short barreled (14.5″) M4 carbine. And if you also want the benefits of M4 feed ramps, make sure they’re on both the barrel extension and the upper.
M4 Feedramps - thearmsguide.com
Here’s the difference between M4 feedramps and other variations. I always opt for M4 feedramps in my barrel extensions and upper receivers. Photo courtesy of ar15barrels.com

 

  • If I have the option of selecting a barrel chambered in either 5.56 or .223, I always opt for 5.56. This allows me to shoot either round without concern. The quick and dirty is a barrel chambered in 5.56 can shoot either .223 Rem or 5.56 NATO, but a barrel chambered in .223 can only safely shoot .223. If you are interested in the full story on that, check out Destinee’s article here.
  • I generally opt for chrome lined and cold hammer forged barrels for AR-15s for their durability. The cold hammer forging process makes for a strong barrel that can usually withstand a high round count. The chrome lining, which is highly corrosion resistant, means that frequent cleaning is less crucial. In my experience, with chrome-lined barrels, I can fire over 800 rounds without cleaning the bore.
  • For a significant boost in accuracy, I opt for custom button rifled stainless steel barrel (non-chrome-lined) chambered in .223 Wylde. For a piece like this, I go to CIV Tactical. They hold high standards for their handmade barrels and each barrel they create is custom made to order. An included headspace matched bolt is an added bonus. This type of barrel would benefit from “barrel break-in” and proper cleaning habits.

The Bottom Line

In range conversations, online forums and other media, I think that the term “mil-spec” is often tossed around in the wrong context. Are military specifications important when referencing dimensional values and tolerances? Yes they are. Is the term mil-spec as important when concerning aesthetics, finishes, billet or forged aluminum, etc.? Not really. As long as you can utilize any reputable lower parts kit and other components without any issues, have fun! If you spend money on a quality bolt carrier group and barrel, you’ll be set. What  build components are most important to you?