AR-15 Buying Guide: Technical Details

My advice so far has simply been to purchase a basic configuration from quality manufacturers and let them work out the details. However, at some point we all have to start understanding what we are looking at when shopping around. What follows is a breakdown of ads from various AR manufacturers. I will try and explain selling points as best I can, as well as what you should be looking for. In the last five years or so, most manufacturers (even the not-so-great ones) have been shifting their production runs to follow the infamous “chart” of quality AR-15s.

Remember that just because the manufacturer checks off boxes for materials or components, it doesn’t mean that the parts are dimensionally correct. Contracted specification and tolerances still matter. A part can be made from all of the right materials, but if the part has a wide tolerance (meaning it deviates from whatever specification it is supposed to), it can still cause malfunctions.

Ok, let’s start looking at some ads. The first one up is a Bravo Company Manufacturing (BCM) A4 rifle, as sold at G&R Tactical. I started with BCM because I know they are an quality manufacturer who sticks as tightly to accepted specs as possible without blowing your budget.


BCMGUNFIGHTER™ Mod 4 Charging Handle
Receivers Machined from Aluminum Forgings 7075-T6
Receivers Hardcoat Anodize per MIL-A-8625F, Type III, Class 2
M4 Feed Ramp Flat Top Receiver
Detachable 600M Carry Handle Rear Sight
T-Marked Upper Receiver

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

Rifle Length Gas System

Bolt Machined from Mil-Spec Carpenter No. 158® Steel HPT Bolt (High Pressure Tested/ Proof)
MPI Bolt (Magnetic Particle Inspected)
Shot Peened Bolt
Tool Steel Extractor

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

Receivers Machined from Aluminum Forgings 7075-T6
Receivers Hardcoat Anodize per MIL-A-8625F, Type III, Class 2
Low Shelf for RDIAS Installation – Installation of a registered drop-in auto sear allows weapon to fire on full automatic
Low Shelf for Accuwedge Use – Accuwedge Stabilizes fit between upper/lower receiver
Un-notched Hammer Compatible with 9mm Use
Fire Controls Marked SAFE and SEMI
Magpul MOE Enhanced Trigger Guard
BCM® Milspec 7075-T6 Receiver Extension
A2 Fixed Stock

7 lbs 12 oz

That is a pretty thorough list, which is why I started with it.

Starting with the receivers, note that they are both machined from forgings of 7075 aluminum. This process means that the aluminum was hammered into shape by a large forging press, and then all of the cavities and holes were machined out. The alternative to forging is billet, where a block of aluminum is placed into a CNC machine and “sculpted” 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 better fit of components and better accuracy. Billet can also be carved into more fancy designs.

7075 should be your go-to for AR15 receivers. The other common alternative, 6061 aluminum, was the original specification in the early 1960’s due to easier machining (7075 is more than 50% stronger). Experience in Vietnam showed that forgings of 6061 were extremely prone to corrosion in humid environments. Daniel Watters’s excellent history of the 5.56 cartridge mentions examples of takedown pivot holes completely rusting through in as little as three months.

The T6 refers to the type of heat treatment used on the aluminum.

The BCM receiver extension, also known as the buffer tube, is also made from 7075-T6 aluminum. Not a lot of manufacturers will specify what their buffer tubes are made from. In many cases, less expensive rifles may say the receivers are 7075, but the buffer tube may be 6061 in order save some money.

The specs reference M4 feed ramps on both the upper receiver and the barrel. M4 feed ramps are a more recent development that came about in the 90s. They came about in order to increase the feeding reliability of the M4 carbine when fired fully automatic. The original AR-15 system, and its rifle feed ramps (which exist only on the barrel extension), was not designed to operate at the higher pressures and cyclic rates of the short carbine system. The bolt carrier did not always get a firm hold of the next cartridge for feeding, which resulted in malfunctions. To compensate, the feed ramps were extended to run from the barrel extension onto the upper receiver. Note that this development really does not affect civilian shooters with semi-automatic only weapons, but the perceived increase in reliability meant that the M4 feed ramp pattern became ubiquitous. The full size M16A2 and A4 still use the old rifle ramps, but most civilian ARs come with M4 ramps. What you need to keep in mind is that the ramps on the barrel and receiver should be matched. It is OK to use an M4-ramped barrel extension on a rifle-ramped receiver; but using a rifle-ramped barrel extension on an M4-ramped receiver will cause malfunctions. This latter combination results in a distinct “lip” that overhangs the receiver’s feed ramp, and the nose of bullets will get caught in it.

Comparison of various AR feed ramp configurations

‘T-Marked’ simply means that there are markings to let you know what part of the rail you are using. The intent is help keep you be consistent when you attach and detach various accessories.

The barrel has these 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 the 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. If you want to deviate from a standard mil spec barrel, check out my post concerning AR-15 barrels. Barrels are an instance of the ‘spec’ being just a commonly accepted standard, you can buy great barrels for less money that follow a different spec sheet.

High pressure testing and magnetic particle inspection 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. As you will see later, this is also done on the bolt for the same reason. The intent is to make sure that the part will not fail you prematurely. Some manufacturers, like BCM, will do this on every bolt and barrel they sell- which raises costs. Others will do it on a few barrels from each batch. Others might not do it at all. I have seen one manufacturer state that doing 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 state that they no longer do these tests out of concerns that the test actually shortens the life of the component.

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, since it allows the use of cheap surplus ammunition. However, .223 chambers are slightly more accurate- so keep that in mind. There are various hybrid chambers out there, like .223 Wylde, that can comfortably shoot both.

The barrel bore and chamber are 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.

url.jpgThe F-Marked and taper pinned front sight base is another indicator. 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 intended 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 has two grooves drilled on the bottom, and 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 will 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 will 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 switch to a different style of gas block or front sight, you will have to deal with the grooves from drilling the old one. This is why people with fixed front sight bases, as I advocate for new buyers, will turn to shaving down the front sight instead of replacing it with another gas block entirely.

Manganese phosphate is simply an outer coating to give the barrel its unique black look and offer some environmental resistance. There are lot of coatings out there, so don’t get hung up on any particular one. Something to look for, though, is that the manufacturer actually put the coating on the barrel before they pinned the FSB in place. Inexpensive barrels may be bare steel under the FSB, and you would never see it.

The bolt carrier group has these specs:

Bolt Machined from Mil-Spec Carpenter No. 158® Steel HPT Bolt (High Pressure Tested/ Proof)
MPI Bolt (Magnetic Particle Inspected)
Shot Peened Bolt
Tool Steel Extractor
BCM® Extractor Spring
Black Extractor Insert

Chrome Lined Carrier (AUTO)
Chrome Lined Gas Key
Gas Key Hardened to USGI Specifications
Grade 8 Hardened Fasteners Key
Staked Per Mil-Spec

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 will typically go the cheapest path that meets their reliability needs. Lewis Machine and Tool (LMT) has written that Aermet is a 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 spec was written). The bolt carrier is typically 8620 steel (this isn’t listed).

The bolt should be shot peened, which increases the resistance of the bolt to fatigue. Many people suggest only buying bolts that have been individually high pressure and magnetic particle inspected, as with barrels. The same thing I said about barrels applies here.

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. 

When it comes to bolt carriers, there are very few reasons you should not be running an M16 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 and are really only used today for lightweight operation in competition rifles- and even those styles (like the JP LMOS) are much better designed.


You will also notice that the BCM advertises 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 (not required on a rifle buffer tube, but you will see that the BCM carbine advertises staking on the castle nut as well).


Because I used an A4 rifle in this example, something that is not listed in these specs is the mil-spec receiver extension used on carbines (the carbine link I posted has it, though). Receiver extensions should be 1.14″ in diameter, also known as “mil spec” (as opposed to “commercial”). 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.


The rest of the listed specs on this rifle are pretty self-explanatory, so I won’t go into them. What I’ve presented here is pretty much what you should be looking for as a base standard. What I want to do now is post some spec lists from other manufacturers so you can sort through them.

First up is a fairly new company on the market making a name for themselves, Sionics Weapon systems. This is their Patrol Rifle Zero, which fits in the same category of basic carbines that I advocate for first time buyers.

Here are the specs

Configuration: M4 Style Carbine
Upper Receiver: 7075 T6 with M4 Feed Cuts and T-Markings
Barrel: 16” Medium Weight Contour, CMV 4150, 5.56 NATO Chamber, 1:8, Chrome Lined Barrel and Chamber, Air-Gauged, Radiograph and MP Inspected
Gas System: Mid-Length, Direct-Impingement
Front Sight: F-Marked Front Sight Base
Rear Sight: A2 Aluminium – Manual Folding
Muzzle Device: A2 Flash Suppressor
Bolt Carrier: 8620 M16 Profile Carrier. NP3 Coated
Bolt: Carpenter No.158, HP/MPI Tested, Black Insert and Sprinco 5-Coil Extra Power Extractor Spring. NP3 Coated
Charging Handle: USGI
Lower Receiver: Forged 7075-T6 Aluminum
Trigger: SIONICS Enhanced Mil-Spec
Lower Receiver Parts: All standard Mil-Spec parts
Receiver Extension: 6 Posistion Mil-Spec (Dry Film Lubricated)
Buffer: Carbine “H” Buffer
Stock: B5 Systems SOPMOD Bravo
Grip: ERGO*
Hand Guard: Magpul MOE
End Plate: IWC QD
Extras: Magpul 30 Round Magazine
Rifle Weight – 6 lb. 13 oz.

How many items hit the specs? It looks to me that the important ones are covered, with some other flashy things thrown in (NP3 coating, for instance). Some things are not mentioned, however- such as the receiver extension material or staking. For these items, you have to rely on reputation of the company, emails with specific questions, or looking at photos of their products.

Let’s look at a ‘budget’ AR that someone at the local gun shop might be interested in if they didn’t know any better. This is a DPMS Panther Arms sold at Cabela’s for $599.

7075-T6 upper and lower receiver
16″ 4140 Chrome lined barrel with 1/9 twist
Commercial buffer tube

Not a lot of information there, honestly. But you can see that they use a cheaper grade of barrel steel with a 1/9 twist. The commercial buffer tube is unfortunate, since it will not be compatible with most of the stocks on the market. No information is given about the internals, what they are made from, and how they are assembled.

Here is another common one, Smith & Wesson M&P 15 Sport II ($650)

The Smith & Wesson Model M&P15 Sport™ 5.56mm NATO Semiautomatic Rifle features an adjustable A2 Post front sight and an adjustable Dual Aperture rear sight. The rifle is designed with a 6-position telescopic stock and a 4140 steel barrel with a Melonite® finish, as well as an aluminum receiver with a hard-coat black anodized finish. 30-round detachable PMAG® magazine.

Features and Benefits
Semiautomatic firing
6-position telescopic stock
Adjustable A2 Post front sight and adjustable Dual Aperture rear sight
Chrome-lined gas key and bolt carrier
Made in USA
4140 steel barrel with a Melonite® finish and a 7075 T6 aluminum receiver with a hard-coat black anodized finish

Again, a cheaper 4140 barrel. The barrel is 1/8 twist and Melonite, which are both fine. Also, no mention of internals, materials, and manufacturing methods. Now, my own research shows that S&W does do a lot of the right “stuff” with their rifles, they just don’t advertise it like the enthusiast/professional grade companies do.

One more. This is a Del-Ton Inc (DTI) carbine. It retails on their web site for $753.

  • Chrome Moly Vanadium
  • M4 Feed Ramps
  • 16″ Length
  • 1×9 Twist
  • A2 Flash Hider
  • Manganese Phosphated
  • Phosphated under Front Sight Base
  • Taper Pins on F-Marked Front Sight Base
  • Threaded Muzzle
  • M4 Profile


  • 5.56 X 45 mm
Bolt And Carrier:
  • Phosphated 8620 Steel Carrier Assembly
  • Carpenter 158 Bolt HPT/MPI Tested
  • Heat Treated and Plated
  • Mil-Spec
  • Chrome Lined Carrier Interior
  • Carrier Key – chrome lined, attached with Grade 8 Screws
  • Properly Staked & Sealed Gas Key
  • Carbine Length
  • Aluminum Delta Ring
  • Single Heat Shield
Upper Receiver:
  • Forged 7075 T6 Aluminum
  • Flat Top With M4 Feed Ramps
  • Hard Coat Anodized
  • Mil-Spec
  • Ejection Port Cover and Round Forward Assist
  • Right Hand Ejection
  • Bore’s surface is coated with dry film lube, over the anodized surface
Lower Receiver:
  • Forged 7075 T6 Aluminum
  • Hard Coat Anodized
  • Mil-Spec
  • Aluminum Triggerguard
  • Semi-Auto
  • Aluminum Mag Catch Button
  • M4 5 Position
  • Reinforced Fiber
  • Mil-Spec Buffer Tube
  • 6.4 lbs Empty

From the spec list, this actually looks like it hits all the important points (except for 1/9 twist and an unknown blend of CMV on the barrel steel). However, my question then becomes: how are they selling them cheaper? This is where you have to make that judgement call. They may check all the right boxes, but what about the contracted specs of those parts, the assembly process, and the quality control methods?

I want to note that I’m not saying these brands are bad. I’m just pointing out that they either don’t follow the accepted standard or at least don’t advertise that they do. All of the companies I’ve found that do stick to the standards, or exceed them, charge about the same price for their weapons (Colt, BCM, Sionics, Daniel Defense, etc.). Companies that charge significantly less tend to change what goes into their weapons. How you decide from there is up to you.

Thanks for reading!


6 thoughts on “AR-15 Buying Guide: Technical Details”

  1. The -T6 in 7075-T6 refers to the type of heat treatment that the alloy gets. It has nothing to do with surface coatings.

    Other than that, good article.


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