AR-15 Technical Details: How to Read the Spec Sheet

You know how that “teach a man to fish” saying goes? I always thought it was a little pithy, but it wasn’t wrong. I’ve always had the mindset that teaching people how to do something was more productive in the long run than just doing it for them. This is one of those posts. I’m going to review common AR-15 specs and explain what they mean.

One of the hardest things with writing about guns, and AR-15s especially, is that a lot of people just want to be told what to buy. It gets even harder when they’ve already made up their minds about what they want and they’re really only looking you to validate it.

I’ve indulged a little with the “just tell me what to get” crowd regarding a few things here or there. But it felt…artificial.

Therein lies the rub: they are really just my preferences. There are a lot of great rifles and barrels out there, and the subtle differences between them are often purely academic.

In this post, I want to dig into the technical specs you should look for when shopping around. If you’re new, I want to help you better interpret what it is you’re reading. Marketing teams are very good at throwing up smoke screens to misdirect you.

When Rob_S put out the original “chart” in the late 2000s, it became the de-facto standard for shopping around. Companies quickly caught on and started marketing their wares while making sure to hit the key points.

Before I get going here, I want to emphasize that just because a company lists the right spec on their sales copy, there is still a lot of room for things to go wrong. That’s where quality control standards come in, and that’s a difficult thing to market.

Enter the Example

To start this off, let’s look at a quality example. This is the advertisement for Bravo Company USA’s BCM A4 model. I’ve bolded some key phrases worth remembering and grouped related AR-15 specs together.

The Barrel

• M4 Feed Ramp Barrel Extension
• USGI 5.56mm NATO Chambers
  
• Independently Certified Mil-Spec 11595E Barrel Steel
  
• Chrome Lined Bore and Chamber
  
• Manganese Phosphate Barrel Finish on entire barrel
  
• Mil-Spec F-Marked Forged Front Sights
  
• Taper Pinned Front Sight Base
  
• USGI M16A4 Government Profile Barrels with 1/7 twist rate
  
• HPT (High Pressure Test, per Milspec) Barrels
  
• MPI (Magnetic Particle Inspected, per Milspec) Barrels

Bolt and Bolt Carrier

• Bolt machined from Mil-Spec Carpenter No. 158® steel
• HPT Bolt (High Pressure Tested/ Proof)
• MPI Bolt (Magnetic Particle Inspected)
• Shot Peened Bolt
• Chrome Lined Carrier (AUTO)
• Chrome Lined Gas Key
• Gas Key Hardened to USGI Specifications
• Grade 8 Hardened Fasteners Key
• Staked Per Mil-Spec
• Tool Steel Extractor
• BCM® Extractor Spring
• Black Extractor Insert

Upper and Lower Receiver Housings

• BCMGUNFIGHTER™ Mod 4 Charging Handle
• Standard Rifle Length Gas System
• M4 Feed Ramp Flat Top Receiver
• T-Marked Upper Receivers
• Receivers Machined from Aluminum Forgings 7075-T6
• Receivers Hardcoat Anodize per MIL-A-8625F, Type III, Class 2
• BCM® A2 Rifle stock w/ buttplate and metal trap door.
• Magpul MOE Enhanced Trigger Guard
• Low Shelf for RDIAS installation
• Low Shelf for Accuwedge use
• Un-notched Hammer compatible with 9mm use
• Fire Controls marked SAFE and SEMI

This particular rifle comes with a 600m detachable carry handle and a magazine. The total package comes in at a weight of 7 lbs 12 oz.

So How do You Read AR-15 Specs?

Let’s start with the foundation: the receivers.

Both the upper and lower come from machined from forgings of 7075 aluminum. To produce a forged receiver, a large forging press presses a block of 7075 aluminum into the desired shape. The resulting shaped hunk of metal then passes through a machining process where all of the cavities and holes are created.

The alternative to forging is billet, where a solid block of aluminum goes into a CNC machine. The computerized machine “sculpts” the block into a final design. Forging may produce a slightly stronger receiver due to the compression of metal grain, similar to the slightly increased strength of hammer forged barrels.

Billet receivers may have tighter tolerances, leading to a better fit of components and potentially better accuracy. Billet will usually be slightly heavier to make up for the lack of compressed metal grain. The difference is very small, likely about an ounce, so it’s not worth making a buying decision over weight differences.

Receiver Material

Just about everyone should focus on 7075 for their AR-15 specification.

The other common alternative, 6061 aluminum, was the original specification in the early 1960s since it’s easier to machine and extrude. Experience in Vietnam showed that forgings of 6061 were extremely prone to corrosion in humid environments. Daniel Watters’s history of the 5.56 cartridge hosted at Loose Rounds mentions the takedown and pivot pin holes rusting through in as little as three months.

7075 is more than 50% stronger in comparison.

The T6 in 7075-T6 refers to the type of heat treatment used on the aluminum. It involves heating, quenching, and heating again. This strengthens the aluminum alloy considerably.

It’s not listed for the above rifle’s specs, but BCM also uses 7075-T6 receiver extensions, otherwise known as the buffer tube. They list this specification on their manufacturing website.

Not many manufacturers specify their buffer tube materials. In many cases, such as with less expensive rifles, the manufacturer may say the receivers are 7075-T6, but the buffer tube may be 6061 in order save some money. I don’t have any reliable data on whether or not the receiver extension material makes much difference, but all the benefits of 7075 still apply as far as strength and corrosion.

Receiver Extension Dimensions

Because I used an A4 rifle in this example, the dimensions of the receiver extension aren’t mentioned.

AR-15 carbine receiver extensions should be 1.14″ in diameter, also known as “mil-spec” in the advertising world. The opposite is the “commercial spec.”

I don’t see commercial extensions very often anymore, and I think the industry has more or less settled on the “mil-spec” tube. If going the rifle buffer route, then disregard these dimensions and simply worry about the material.

Upper Receiver and Barrel Fitment

Let’s talk about feed ramps. The BCM rifle above mentions M4 feed ramps on both the barrel and the upper receiver.

This is important.

The M-16 specification called for feed ramps only on the barrel extension alone. This worked fine with the relatively lower pressure rifle gas system and its more moderate cycle rate. In the early 1990s, there were noted issues with the M4 Carbine’s ability to consistently feed on fully automatic.

The increased gas pressure from the carbine gas system along with an increased cyclic rate led to the development of “M4 Feed Ramps.” This style of ramp extends from the barrel extension onto the upper receiver.

For us regular Joes, which one we use doesn’t really matter. We aren’t going fully automatic, so we don’t really benefit. That said, you need to pay attention to the combination of rifle and carbine ramps between the barrel and receiver.

A rifle-ramped barrel on a carbine-ramped upper leaves a distinct lip that will jam the rifle. In this configuration, the tip of the bullet gets stuck under the lip. Your options are rifle-to-rifle, carbine-to-carbine, or carbine-barrel to rifle-receiver.

To date, the M16A2 and A4 rifles in the inventory still use the rifle ramp configuration. Nearly everything is using M4 Ramps, including most of the civilian market.

The Barrel

I’ll be brief here and hit the highlights. If you want to know more about barrels, I highly suggest checking out my article on AR-15 barrels.

These were the specs:

• Independently Certified Mil-Spec 11595E Barrel Steel 20″ USGI M16A4 Government Profile Barrel
• 1/7 Twist Rate
• HPT (High-Pressure Test, per Milspec) Barrel
• MPI (Magnetic Particle Inspected, per Milspec) Barrel
• M4 Feed Ramp Barrel Extension
• USGI 5.56mm NATO Chambers
• Chrome Lined Bore and Chamber
• Mil-Spec F-Marked Forged Front Sight
• Taper Pinned Front Sight Base
• Manganese Phosphate Barrel Finish on Entire Barrel

The military spec for barrel steel is a 4150 Chrome Moly Vanadium (CMV) alloy, specifically known as Mil-B-11595E. This blend is well understood, having been originally spec’d in 1966. 4150 reflects better hardness and heat characteristics over the slightly cheaper 4140 blend, but the average shooter is not likely to push a barrel to the types of temperatures that would seriously cause problems.

Keep in mind that just because a manufacturer says the barrel is 4150 or CMV, it doesn’t mean it meets the standards of Mil-B-11595E. BCM, for example, goes out of their way to have their barrel steel certified to the spec. That increases costs, but proves a known quality.

Quality Control

High-pressure testing (HPT) and magnetic particle inspection (MPI) mean that the manufacturer fired a high pressure “proof load” through the barrel, then used a process to look for imperfections and weaknesses that may lead to later failure.

The intent is to make sure that the part will not fail you prematurely.

Some manufacturers, like BCM, do this on every bolt and barrel they sell. Of course, that raises costs.

Other manufacturers might do it on a few barrels from each batch. Yet others might not do it at all.

I have seen one manufacturer state that the value of MPI is greatly diminished without doing the HPT first, so keep that in mind when you see only one or the other. I have also seen another manufacturer of very high-quality rifles, KAC, state that they no longer do these tests out of concerns that the test actually shortens the life of the component.

Chamber

This rifle has a 5.56 NATO chamber. What you need to know is that a 5.56 rifle can fire 5.56 or .223, but the same is not true in reverse.

5.56 NATO is a higher pressure round than .223.

A .223 chamber may not have enough space for the extra gas of 5.56 and could result in a catastrophic failure. For that reason, most people look for a 5.56 NATO chamber in their AR-15. It allows tolerance for cheap surplus ammunition.

However, .223 chambers are slightly more accurate due to the tighter specs.

At least, that’s what the lore says. Lucky Gunner did a great study in 2012 looking at the truth to all of this. The reality is that NATO measures the pressure of ammunition differently than SAAMI does. That’s the heart of the difference.

There are various hybrid chambers out there, like .223 Wylde, that can comfortably shoot both.

Chrome Bore and Chamber

The barrel bore and chamber is chrome lined.

Again, this is the military standard, but you don’t need to follow it if you go for a different barrel material or lining type. Melonite (also called salt bath nitride, QPQ, and other names) is a surface conversion/hardening process that provides good corrosion resistance without the loss in accuracy of chrome.

However, be advised that it is a fairly new process, and the temperatures that the barrel must be heated to are pretty high. The long term effects of heating the barrel in such a manner are not well understood.

Buy quality and let the manufacturer worry about how it gets made.

F-Marked Front Sight Post

The F-Marked and taper pinned front sight base is another indicator of quality.

An F-Marked base means that the shelf of the front sight is the correct height for detachable carry handles. Keep in mind that nearly all rear sights made today are designed to work with F-marked front sight bases. If you don’t have one, then you will have to adjust the front sight pin high enough that the base of the pin may become exposed.

Taper pinning means that the barrel and front sight base are drilled on the bottom to match The front sight is secured by wedging steel pins through the sight and grooves. This is the most secure way to attach a front sight or gas block.

There are two downsides to taper pinning, though.

First, any removal of material from the barrel in such a non-uniform way could result in changing the harmonic vibration of the barrel during firing, which means decreased accuracy. That is why a lot of match rifles have clamp-on gas blocks.

Secondly, taper pinning is unique to the front sight or gas block you are using. You cannot simply remove the base and put another one in its place. Quality manufacturers not only pin the FSB in place, but they also make sure that it is straight. Some budget brand ARs have the FSB pinned, but it will be crooked, and there is no fixing it once the pin holes are drilled.

If you want to swap to a new gas block, I suggest shaving down the FSB rather than trying to refit a different one in its place.

The Bolt and Carrier

The bolt is made from Carpenter-158 tool steel. Again, this is the contracted military specification, but that doesn’t mean it’s necessarily the “best” option.

Always remember that government contracts typically go the cheapest path that meets their reliability needs. Lewis Machine and Tool (LMT) wrote that Aermet is better steel for bolts, as it is 2.5 times stronger, but it’s also quite a bit more expensive and wasn’t available when the original specs came about.

he bolt carrier is typically 8620 steel, though this isn’t listed above.

The bolt should be shot peened, which increases the resistance of the bolt to fatigue. This bolt has also undergone HPT and MPI testing, as with the barrel. The same pros and cons apply.

A tool steel extractor simply means that the extractor is better than soft pot metal. The more important part here is the better springs and inserts, which provide a stronger “grab” on the base of the brass for positive ejection.

Auto Carriers

When it comes to bolt carriers, there are very few reasons you should not use an M-16 AUTO carrier. That means that the carrier is the same dimensions and weight as the original select fire design.

This does NOT mean that your rifle will fire fully automatic. The AUTO refers more to the weight and dimensions. You can see the difference below.

The lighter commercial AR-15/semi auto carriers can negatively affect reliability. You should only consider them for lightweight operation in competition rifles- and even those styles (like the JP LMOS) are much better designed.

Staking

The BCM advertisement mentions staked gas carrier keys.

Staking is important!

Staking is the process of indenting fastening screws in certain key areas to prevent the fastener from backing out. You should not see anyone using Locktite or other thread lockers on these areas- they should be staked into place. This includes the bolt carrier gas key, two screws staked twice each, and the castle nut of a carbine buffer tube

Wrapping Up

I could go on and grab some ads from other web sites for comparison, but I want to leave that up to you.

So here’s your project, go find an AR-15 from a company you like and post it in the comments. Let’s compare what that company says about their product against the expected standards.