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AR 15 Upper: Build, Buy, Better

If the name of the game is modularity, the AR-15 certainly stands on the podium of best examples. The split receiver makes swapping calibers as easy as pulling two pins and replacing the AR 15 upper, with each one retaining its own zero, so long as you don't remove the optic. Budget, custom performance, and the fun of it drive most motivation for building versus buying your own AR upper, and that landscape continues to evolve. In both 2013 and 2020, some opted to build their own out of parts availability before even that source began to dry up. We've optimized a few firearms here and there, so let's take a look under the rail at what goes on in the AR 15 Upper.


A ravine divides AR owners. On the one side, you have those dedicated to traditional direct impingement function, on the other, the piston-operated tribe stands resolute, looking down with a smug sense of superiority. The similarities shared between these two groups should be the grounds for a long and fruitful relationship. Instead, we must endure the collateral damage of yet another needless conflict.

In reality, the rivalry between piston-operated and direct impingement AR 15 upper receivers is mostly in good fun. The difference between the two comes down to parts, preference, and situational influences.

A complete AR 15 upper consists of a barrel, muzzle device, gas block, gas tube or piston system, rail, receiver, bolt and bolt carrier group, and charging handle. A majority of the stages of the cycle of operations (feeding, chambering, locking, firing, unlocking, extracting, ejecting, and cocking) take place in the upper.

When the trigger is pulled, the hammer is released to strike the primer of the cartridge seated in the chamber. As the expanding gasses push the projectile out the barrel, they eventually pass the gas port, where some pressure is vented through a gas block and either through a tube or into a piston. This energy is routed into the top of the bolt carrier group, which begins to move rearward. The cam pin rotates the bolt, unlocking the lugs as the extractor holds onto the rim of the casing, extracting it until it has the clearance to be ejected from the AR 15 upper receiver. The rearward motion of the bolt carrier group pushes the hammer down, resetting or cocking it. The compressed buffer spring reverses the motion of the bolt carrier group, and as the bolt face passes over a loaded magazine, it strips a round, pushing it into the chamber via the feed ramps. As the bolt enters the star chamber, it begins to rotate, resulting in a locked position with lugs touching.

AR 15 Upper Parts

By now, each component of the AR 15 upper has received an aftermarket makeover, bending our understanding of what a “stock” firearm means anymore. From coatings, to materials used, to cuts, shapes, and the ability to interface with lights, lasers, and foregrips, even the upper receiver has undergone multiple forms of evolution, resulting in distinct strains.

When looking at recievers, lowers get a whole lot more attention than AR 15 upper recievers, except when you are the BATFE trying to remedy your past definitions. Much of this comes down to the variety of features integrated into lowers, such as ambidextrous controls. While most AR uppers come in either the forged or billet varieties, a fair amount of personal choice beyond appearance is available. Some have deleted the forward assist and dust cover to save weight or manufacturing time, others have reinforced sections, or have been skeletonized to save weight. JP Enterprises even makes a dual-charging AR 15 upper with a folding side-charging handle. For left handed people, Stag Arms has long produced an upper that ejects spent brass out the left-hand side.

In RECOIL #59 we included a list of firearms components that we considered improvements to MIL-SPEC versions of the parts, here's an excerpt regarding the BCM Mk 2 Upper:

BCM Mk 2 Upper

Beyond aesthetics, the upper receiver of the AR platform hasn’t received nearly as much attention from manufacturers as other components. BCM stepped in to remedy this with their Mk 2 upper receiver.

Chopping off the AR’s carrying handle to create the now-ubiquitous flat-top upper resulted in an incredibly versatile platform with the ability to easily attach optics and accessories. However, it also removed a source of structural integrity, in particular leaving the ejection port as a key point of weakness. 

BCM Mk 2 Upper

BCM carefully refined the upper to move mass to where it’s most needed, increasing rigidity by about 30 percent while only adding 0.3 ounce to the total weight. They shrunk and relocated the forward assist and brass deflector and added more mass around the ejection port on the right side (60 percent more, in fact), and the front of the upper on the left side. These reinforcements reduce flex and enhance accuracy potential while reducing wear on the bolt and barrel lugs. Meanwhile, the Mk 2 upper still takes standard USGI parts.

BCM Mk 2 Upper

Additionally, expansion chambers in the charging handle track and small vents help redirect gas away from the shooter’s face when shooting suppressed. And moving the forward assist tidies up the rear of the upper, making it easier to use ambidextrous charging handles and sling attachments on the endplate. 

BCM’s Mk 2 upper is a thoughtful and elegantly designed upgrade.

MSRP: $165

Light Weight Parts

With each build, the operating conditions drive parts selection. Where some components undergo a redesign to strengthen a weak point, others undergo surgery or are re-made with different materials to increase or reduce weight.

Proof Research Carbon Fiber AR 15 Barrel

Even in 2022, Carbon Fiber still carries an air of Space Age aesthetic. Beyond looks, however, it has expanded the possibilites of various components by an impressive strength-to-weight ratio. Proof Research barrels often sit as the heart of many bolt action precision rifles, but they also produce carbon wrapped stainless AR 15 style replacements in a variety of lengths and twist rates. With up to a 64% weight difference from traditional barrels, the carbon fiber wrap also helps deal with the heat generated from firing.

Proof Research Carbon Fiber AR 15 Barrel

Lengths Available in 223 Wylde: 11.5, 14.5, 16, 18, and 20 inches
Other Chamber Options: .204 Ruger, 224 Valkyrie, 22 Nosler, 6 ARC, 6.5 Grendel, 300 Blackout, 6 Creedmoor, 6.5 Creedmoor, and .308 Winchester.
MSRP: $980

Superlative Arms Titanium Adjustable Gas Block, Bleed Off

Titanium is an element, and steel is an alloy, and that should settle the argument between materials. The property of each determines each to be better suited for certain components, based on the environment. Generally speaking, suppressing an AR-15 increases the pressure within the gas system, and as a result, increases the violence at which the bolt and carrier group move throughout the firearm, wearing the friction points at a greater rate. An adjustable gas block can mitigate some of this by restricting the amount of gas vented off the barrel.

superlative arms titanium adjustable gas block

Superlative Arms utilizes a patented bleed-off system to lower the gas pressure, which differs from the norm. Often a screw is used to adjust the gas flow, where the body itself partially covers the port. Superlative Arms offers a different option, where pressure above that necessary to cycle the bolt vents forward of the rifle, under the rail, and away from the shooter.

superlative arms titanium adjustable gas block

The titanium version saves .6 ounces, which in addition to a lightweight rail and barrel quickly add up. An adjustable gas block allows the user to tune their firearm to their needs: more gas for more reliability with varying ammunition types, or less gas for less felt recoil, riding the lightning edge of gas pressure will probably never go away.

The Superlative Arms Bleed Off Style Adjustable Gas Block can be set in two different ways. By turning the adjustment screw, the user can choose between a bleed-off setting or a traditional gas-restriction position.

Diameter Options: .625, .750, .875, .936
Weight: 1.08 ounces
MSRP: $210

Performance Enhancers

Not everything on an AR 15 upper comes down to weight. The conditions imposed on the bolt and bolt carrier group induce increased wear, putting the integrity of the system as a whole at risk if not maintained. The muzzle end of an AR 15 often comes down to personal preference, or is determined by the user's suppressor of choice.

JP Enterprises Full Mass Bolt Carrier Group and EnhancedBolt

JP Enterprises has undergone great lengths to produce a sewing machine of an AR-15 resulting in their popularity among the world's best competitive shooters. The FMOS or full mass Bolt Carrier Group by JP Enterprises makes major and minor adjustments to the traditional BCG. Bolt tilt takes place as the pressure expanding from the gas tube disproportionally impacts the top end of a bolt carrier. In a loose AR 15 Upper Reciever, this small tilt of the bolt carrier group induces additional wear over time, eventually requiring parts replacement.

JP Enterprises Enhanced Bolt Carrier Group and Silent Capture Spring System
The Enhanced BCG pairs well with JP's Silent Capture Spring System.

The JP Full Mass Bolt Carrier Group minimizes this effect by the geometry of its design. A re-assessment of the points of contact between the BCG and the sidewalls of the receiver mitigates bolt tilt, and a Quench-Polish-Quench process applies a smooth, corrosion-resistant surface with natural lubricity.

MSRP: $353

Muzzle Talk

Muzzle devices typically come in 4 flavors: brakes, compensators, flash hiders, and hybrids. On an AR 15 upper, the choice of muzzle device can be a telltale giveaway of the environment that firearm gets used in most often. Flash hiders lean tactical, where compensators and brakes dominate the competition field. As a rule, those who bring a muzzle brake into a shoot house are looked at as rude, as they gently concuss their friends and potential teammates in the confined space. However, in the open air of a competition bay, all that recoil mitigation can pay dividends with practice.

Cox Arms Wulf 5.56
The VG6 Gamma Muzzle Brake includes both side and top ports and comes standard on a Cox Arms Wulf AR Pistol.

Video games sometimes depict a flash hider attachment as a half-value suppressor. This is both correct and incorrect at the same time, for while a flash hider does nothing to reduce the sound of a gunshot, it spreads the expanding gas at the muzzle in a wider pattern to reduce the potential fireball following the bullet. No flash hider completely eliminates the muzzle flash, but when compared to a naked muzzle or compensator, the difference is obvious. Want to give away your position? Fire a short-barreled rifle with a muzzle break at night and they'll see you for miles.

Fazon Flash hider

Over the last decade, we have seen dedicated suppressors directly integrated into AR 15 upper receivers, such as the Daniel Defense ISR. If it weren't for the BATFE's regulation of both short-barreled rifles and suppressors, we'd expect to see a whole lot more of these, considering their health benefits.

An Intro on Barrel Length

The amount of time dedicated to the impact of changing the barrel length on an AR 15 upper would fill pages upon pages, and is much more precise than this breakdown can provide. However, there are generalities: ones worth beginning at and verifying before going too deep into the bank account. After ignition, the bullet gains more velocity as it travels through the barrel, with it's peak speed being right outside of the barrel. Shorter barrels then hurl the projectile at lesser velicities which affects both its trajectory as well as terminal ballistics.

In addition to this, the location and size of the gas port impacts felt recoil, and the function of the firearm. Dwell time refers to the timespan from the moment the projectile passes to the gas port to the point it leaves the firearm. During this spell, gas pressure travels through the firearm, completing the operating cycle. Too small or too close to the barrel for the chosen ammunition load and the firearm will not cycle. Too large and it will induce increased parts wear and felt recoil.

Multiple Uppers Multiple Calibers

One advantage that the platform presents is the opportunity ot have multiple uppers in different calibers, such as one chambered in 5.56, and another in 300 Blackout. An AR 15 Upper capable of firing 300 Blackout rounds can then be outfitted in trajectory-specific optics and a dedicated suppressor for special use purposes. Two dangers present themself however: one to your wallet, and another to your safety.

300 Blackout vs 5.56
Left: 300 Blackout, Right .223 Remington.

Since both 300 Blackout and .223 fit in to the same magazines, mixing them up could result in devastating consequences. Loading a 5.56 round into an upper chambered in 300 Blackout is not completely harmless, but the inverse isn't the same story. Many 300 blackout loads will appear to chamber in a .223 or 5.56 barrel, but when fired, will lodge the projectile at the breach and direct all that pressure towards the shooter. A systemic approach to ammunition storage and marking magazines can help mitigate this possibility, but diligence ultimately makes the difference.

Rail Space

Having the most visual impact on an AR 15 upper, the rail is the point where so much innovation has taken place over the last 40 years. Vietnam era triangle forends gave way to the round ones seen on CAR-15's in Blackhawk Down. Evenutally the plastic guard would be replaced by aluminum, complete with quad picatinny rails until a metaphorical war was waged between M-LOK and Keymod.

As one of the most famous firearms rocking a quadrail, the MK18 made by Daniel Defense features their RIS II Rail with 6 bolts securing it in place to ensure minimal zero shift on a mounted laser.

In the current year, Picattiny, or 1913, remains relevant for both early GWOT nostalgia thanks to some millenials who will not give up their childhoods, and because of the near-univesality of attachments. One of the most important improvements made to the relationship between rail (sometimes akwardly referred to as a heat guard) and the barrel is to increase their independence from one another. With a non-free-floated barrel, which is connected to the rail at more than the barrel nut, when enough pressure is placed on the rail it will shift the point of impact away from the zero.

Outside of this, personal choice inherently plays a major role, while remaining far from the end-all, be-all deciding point. For those using night vision with an IR-Laser, not all rails have the required mounting space to accomodate required equipment, and not every one is created equal when it comes to the effects that heat, wear, and tear have on how well it retains zero. If the rail is likely to move around, bend, or flex under pressure, it could shift the point of aim that a laser produces without the user knowing it.

Build or Buy?

With all that goes into an AR 15 upper reciever, for the individual purchaser, the choice between buying a complete build versus parting out their firearm and putting it together themself comes down to cost and objective. There's something specical to putting together your own firearm, but that comes down to a subjective reverence at best. Still, some considerations come into play when setting out on putting together or purchasing a new AR.

Before amasing a pile of parts, the objective of a first-time AR-15 buyer is to acquire a well-built and reliable firearm for their environment and needs. Many manufacturers can fulfill this first criteria, within a range of cost. What a first-time AR-15 buyer should not do is start as cheap as possible with the plan to upgrade over time. This rarely works out, and results in a pile of money spent exploring new options along the way.

Rather, starting with a reliable AR 15 upper mitigates much of the headache that comes from putting your own one together, but still leaves the door open for customizing your personal firearm one piece at a time. It is much easier to add a new gas block, and much less painfull than buying a new rail, only to discover it is .3 inches too long to allow you to mount your suppressor, a personal experience we can attest to.

More on AR 15 Parts, Accessories, and Buildsheets

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One response to “AR 15 Upper: Build, Buy, Better”

  1. Robert Armstrong says:

    The AR top on the bottom, I have been searching form where I can purchase the hand guard on it. Can you assist me with that? Thanks.
    Love the Magazine.

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  • The AR top on the bottom, I have been searching form where I can purchase the hand guard on it. Can you assist me with that? Thanks.
    Love the Magazine.

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