Optimizing AR Gas System, Buffer System, and Bolt Carrier Mass

If you are unfamiliar with how the AR gas system works, we recommend that you watch this video from 45Snipers:

Newton’s AR

Short Story: The AR operating system is a study in Newton’s laws of motion.

A long time ago, Sir Isaac Newton was playing with his AR and he made a couple of observations that he later coined his “laws of motion.”

The First Law involves inertia (the tendency of an object at rest to remain at rest, and the tendency of an object in motion to continue in motion). The higher the weight of an object, the higher its inertia and, therefore, the more it wants to keep doing whatever its doing.

The Second Law involves momentum (the amount of force required to exert a change in velocity for an object of a given mass). F=m*v. Said in a more applied way, the greater an object’s mass, the more force required to change its velocity.

The Third Law is the law of action and reaction (for every action, there is an equal but opposite reaction). When one object strikes another, both objects experience vector forces that are diametrically opposed. When a hammer strikes a nail, the nail strikes back. When a swimmer pushes off of the wall of the pool, the wall pushes back.

There are two categories of variables in our system: mass and force.

When we ignore external forces (gravitational, applied, normal, air resistance), we are left with three internal forces:

• Pressure: the result of the conversion of chemical energy into pressure (and heat).
• Spring: loading (compression) and unloading (relaxation) of the springs, including the buffer spring and hammer spring.
• Friction: caused by the movement of parts over one another.

The mass that we are concerned with is the that of the moving parts:

• Bolt Carrier Group
• Buffer
• Hammer

A Story About A Gun

Short Story: There’s a lot of sh*t goin’ on.

First off, when the trigger is pressed, the sear on the trigger disengages the hammer. Once released, the hammer rotates forward under the tension of the hammer spring. As it rotates around the hammer pin, the hammer accelerates, until it strikes the firing pin. As the hammer strikes the firing pin, its kinetic energy is transferred to the firing pin, which is driven forward in the bolt carrier and bolt, causing it to strike the primer of a waiting cartridge in battery. The primer detonates and ignites the powder in the cartridge and the train pulls out of the station.

The burning of powder in the cartridge causes conversion of the chemical energy (powder) into rapidly expanding gas and heat. The expanding gas produces sufficient pressure to dislodge the bullet from the neck of the case. The pressure then drives the bullet onto the lands of the barrel, and the projectile begins its journey down the bore and toward its target.

Per Newton’s Third, the force (explosion) pushing the bullet forward is balanced by an equal rearward force. That rearward force (or recoil) will travel into the bolt, transfer to the locking lugs of the barrel extension, then to the upper receiver, then to the lower receiver, and then either to the receiver extension and buttstock (for a shouldered carbine or rifle) or pistol grip (for a pistol).

The recoil DOES NOT drive the bolt carrier group rearward (unless we are talking about PCC or rimfire). The locking lugs on the bolt engage the locking lugs on the barrel extension. You cannot push the bolt carrier group out of battery, and this is really important. If you don’t believe us, open the box below. Otherwise, read on.

The expanding gas is the next to act. As the bullet travels down the bore of the barrel, it will eventually pass the gas port. As it does, the gas will be looking for the path of least resistance and will find a little bit of relief via the gas port. The pressurized gas will pass through the gas port, into the gas block, down the gas tube, and into the awaiting gas key on the bolt carrier. The rush of pressurized gas pressurize a chamber formed by the bolt, carrier, and gas rings. As this chamber is pressurized, the rush of gas will simultaneously push forward on the bolt and push rearward on the bolt carrier, causing the cam pin to ride the path cut in the bolt carrier, causing the bolt to rotate, thereby disengaging the locking lugs of the bolt from the locking lugs of the barrel extension. In essence, the pressurized gas causes the bolt carrier to pull the bolt out of battery.

As the bolt carrier begins to move rearward, three things happen:

1. The bolt carrier meets resistance of the mass of the hammer and the tension of the hammer spring. The bolt carrier will drive the hammer rearward, compressing the hammer spring, until the hammer is out of the way, engages the disconnector, and eventually reengages the sear.
2. The bolt carrier meets resistance of the mass of buffer and the tension of the buffer spring. The bolt carrier will drive the buffer into the receiver extension, compressing the buffer spring against the rear wall of the receiver extension.
3. The spent case is pulled from the chamber by the extractor. Once the neck of the case clears the barrel extension and ejection port, it will be flung out of the upper receiver by the ejector and the force of the ejector spring.

As the buffer spring is compressed, the rearward momentum of the reciprocating mass (buffer and bolt carrier group) will slow to a stop. If you have an over-gassed gun, the buffer may even slam into the rear of the receiver extension; this impact will directly transfer energy to the receiver extension and into the body of the shooter (additional moment of recoil).

Now that the reciprocating mass is no longer in motion, the compressed buffer spring will unload the stored energy by simultaneously driving the reciprocating mass forward and pushing the receiver extension rearward (a little more perceived recoil).

As the reciprocating mass moves forward, it will accelerate. As the bolt passes over the magazine, it will strip the next round and push it over the feed ramps and into the empty chamber. The reciprocating mass will reach terminal velocity as the bolt slams into the barrel extension, beginning a forward impulse. The bolt carrier will continue forward. As it does, the cam pin will ride the groove and rotate the bolt, thereby engaging the locking lugs of the barrel extension. As the bolt carrier reaches its full-forward position, the remaining kinetic energy will transfer into the barrel extension, completing the forward impulse.

That’s a long, heart-wrenching story, we know. But it illustrates the amount of action happening in your gun and shows how complex the interaction of force and mass is. The attributes of each component involved in the process can have a dramatic effect on the process itself.

Beyond the overall weight of the firearm, the things that you can do to tune (or f*ck up) this process include selection of:

• Gas System Length
• Gas Block Design
• Bolt Carrier Mass
• Hammer Spring Strength (we will ignore this because it is negligible)
• Buffer Mass
• Buffer Spring Strength (see our Buffer Springs article to follow)

There are benefits and drawbacks to playing with each of these variables and they all work as a system. You should also be aware that not every combination will work. Let’s walk through how each of these variables works in the AR system.

Gas System Length

Short Story: Longer is (generally) better.

To understand the role of gas system length, we have to understand the pressures occurring within the system. When the primer is struck and the powder begins to burn, the chemical energy stored in the powder is converted into heat and gas. As the particles move faster in a confined space (the cartridge and barrel), the pressure increases. As the bullet (the “cork”) pops out of the neck of the case and moves down the barrel, the molecules have more space to bounce around and the pressure drops. On the path between the chamber and the muzzle, the closer the bullet is to the chamber, the higher the pressure behind it.

There are three important factors relative to gas system length:

1. Latency: The amount of time between cartridge ignition and the flow of pressure through the gas system; not to be confused with bolt lock time, which includes the time it takes to displace the bolt from the locked position (which is not entirely due to gas system length). The shorter a gas system, the less time between ignition and movement of the reciprocating mass (bolt carrier group and buffer) caused by the flow of gas through the gas system. Conversely, the longer the gas system, the longer the delay between ignition and movement of the reciprocating mass. The earlier the reciprocating mass begins to move during the passage of the bullet down the bore, the earlier the firearm will move, and the more impact the moving mass will have on the stability of the gun and the trajectory of the bullet. When speed is important, shorter latency is better (albeit unnoticeably). Where accuracy and precision are important, longer latency is better (noticeably).
2. Dwell Time: The time that the bullet remains in the barrel after it passes the gas port. This is the amount of time that the pressurized gas has to act on the gas system and reciprocating mass, before the the pressure is released at the muzzle. A longer dwell time means that more gas will pass through the gas system, if unrestricted. Excessive dwell time will lead to excessive fouling of the bolt carrier group and upper receiver, as well as more gas in the face of the shooter. Conversely, a shorter dwell time means that less gas will pass through the gas system. Too short a dwell time can lead to short cycling (insufficient force means the reciprocating mass does not travel far enough to the rear to properly cycle) and malfunction.
3. Port Pressure: As explained above, the closer the travelling bullet is to the chamber, the higher the pressure behind it. Consequently, the closer the gas port is to the chamber, the higher the peak pressure experienced by the gas system. Higher port pressure results in a higher and more forceful volume of gas flowing through the gas system. This more forceful flow results in faster acceleration and higher velocity of the reciprocating mass. The added velocity will increase the recoil impulse and can lead to increased stress and wear on the related components. Barrels with shorter gas systems tend to have a smaller diameter gas port to partially compensate for the increased port pressure.

A longer gas system means:

• Longer Latency: This has a positive impact on accuracy and precision for a given barrel length, but will result in a delayed cycling of the operating system.
• Shorter Dwell Time: Less gas in the face of the shooter and less fouling of the bolt carrier group and upper receiver.
• Lower Port Pressure: Less recoil and less wear on the system.

A shorter gas system means:

• Shorter Latency: This has a negative impact on accuracy and precision for a given barrel length, but will result in earlier cycling of the operating system.
• Longer Dwell Time: More gas in the shooter’s face and increased fouling of the bolt carrier group and upper receiver.
• Higher Port Pressure: Because of the more forceful flow of gas*, increased recoil and more wear on the system. Because of the increase in gas volume*, more gas in the shooter’s face and increased fouling of the bolt carrier group and upper receiver.

* Assuming an unrestricted flow.

Gas Block Design

Short Story: Adjustable is more…adjustable…and forgiving.

You have two options when it comes to gas blocks: standard and adjustable (or tunable).

Standard gas blocks have a static path that links the Gas Port in the barrel to the gas tube.

Adjustable (or tunable) gas blocks have a screw or lever that can be adjusted to change the size of the flow path in the gas block. An adjustable gas block allows you to adjust the flow of gas directed down the gas tube by manipulating the amount of resistance to the flow of gas.

An adjustable gas block is a great addition to any setup, though its not technically necessary in every scenario. Regardless of every other variable, an adjustable gas block allows you to fine tune the amount of force applied to the reciprocating mass, which can become useful as you screw around with the weight of the reciprocating mass.

Bolt Carrier and Buffer Mass

Short Story: Lighter reciprocating mass = less recoil, faster cycling, and reduced reliability.

Screwing with the mass of the bolt carrier group and buffer (the reciprocating mass) has a couple of effects to be aware of.

The most obvious impact of a lighter system is the decrease in firearm weight. Obviously, this is a good thing. Nobody wants to lug around extra weight if they don’t need to. However, there is a lot more to the equation when it comes to the weight of the reciprocating mass.

The lighter the reciprocating mass, the lower the inertia. As such, it takes less force to overcome inertia. Practically, it is easier to move the mass from rest, and easier to stop the mass in motion. So what does this mean for your AR?

First, the reciprocating mass will take less pressurized gas to get it moving out of battery (all else being equal). With gas flow and buffer spring weight held constant, this means that a lighter reciprocating mass will begin moving sooner.

Second, the reciprocating mass will accelerate faster. This means that the reciprocating mass will cover the same distance faster. Faster cycling.

Third, the reciprocating mass will decelerate faster. This, again, contributes to faster cycling.

Fourth, the reciprocating mass will carry less momentum, because of the decreased inertial mass. Less kinetic energy translates into a lower recoil impulse.

Faster cycling. Less recoil. This sounds great, right? Faster cycling means you’re ready for the next shot sooner. Lower recoil means a more pleasant experience and faster sight recovery.

Not so fast, cowboy. The same characteristics can have a negative impact, as well.

A lighter bolt carrier is usually lighter because of the materials or a combination of materials and design. These materials (e.g. aluminum) or designs (material removed) can result in a weaker bolt carrier. In fact, aluminum is used in some lightweight bolt carriers and these are considered to be consumable components (i.e. they have a finite lifespan), whereas the lifespan a standard steel bolt carrier is considered practically indefinite. Weaker and less robust is not better.

Faster isn’t necessarily better. If a reciprocating mass travels too quickly, it can overrun the other operating components and lead to malfunctions. If a reciprocating mass reciprocates too quickly, it can literally move faster than the magazine spring can push the next round to the feedlips, resulting in a failure-to-feed. In addition, if you don’t control the force applied to the reciprocating mass (via the flow of gas), the buffer may slam into the rear of the receiver extension (instead of the buffer spring absorbing the kinetic energy), causing stress on the system and sharper recoil impulse. None of this is good.

Because of the decreased inertia, your moving reciprocating mass will have less momentum, so it will be less stable and will be more susceptible to other variables, including changes in port pressure. If you run different loads or intermittently shoot with a suppressor or go to shoot your gun on a cold day, your reciprocating mass will be moving at significantly different velocities, compared to a standard mass or heavy mass system. This inconsistency is not good if your gun is properly tuned.

Because the inertia is lower, the reciprocating mass starts moving sooner, accelerates faster, and gets to it’s destination sooner. The fact that the reciprocating mass is moving sooner is not inconsequential. It means earlier recoil, less stability, and potentially, more impact to the trajectory of the bullet before it exits the muzzle (internal ballistics). This could have a negative impact on accuracy and precision, which is bad.

While the lower inertia means that the reciprocating mass is easier to get moving, it also means that it is easier to stop moving. If your AR is dirty, the resistance/friction in the system is higher. Lower mass means lower momentum when in motion. Easier to stop. A dirty gun with a lighter reciprocating mass will cycle less reliably than one with a standard mass bolt carrier group and buffer. The gun may not reliably go into battery. This is obviously a bad thing.

Along the same lines, the resistance encountered as the bolt carrier group strips the next cartridge out of the magazine can become a problem. Not every magazine feeds round consistently or perfectly. Magazines wear out. Feed lips get deformed. Cartridges are not always presented perfectly. Whatever the reason, not every round will feed from a magazine the same way. A lighter reciprocating mass has less oomf to overcome any inconsistencies. This could lead to failure-to-feed. Which is bad.

So why the hell would anyone go with a lighter weight bolt carrier and buffer? When a fraction of a second is the difference between first place and last place, you look for every advantage you can get. For all of the issues that a lighter reciprocating mass creates, it gives the shooter a slight advantage over someone running a full-mass system (unless it malfunctions). They just need to take extra precautions to ensure their gun runs for the few seconds of a match.

Our Recommendations

Recommendation for Competitive Shooters

If you are looking for the fastest cycle time, the lowest possible recoil impulse, and the fastest possible sight recovery, you want a lighter reciprocating mass and an adjustable gas system. Note that this will come at the cost of reliability and sensitivity to varying conditions: your gun will behave less consistently (or misbehave, altogether) based on cleanliness, lubrication, the ammo you fire, the powder temperature, the ambient temperature, and whatever you have on the muzzle (suppressor vs. brake vs. compensator). If you are a competitive shooter, all of this is probably calculated, tested, and well-controlled. If you’re not a competitive shooter, toeing the line of reliability can get you killed.

Recommendation for Duty/Defensive Shooters

If you are looking for the greatest reliability in the widest array of conditions, you want a standard (or even heavier) reciprocating mass. An adjustable gas system may not be necessary, but can help fine tune the recoil impulse. A heavier reciprocating mass means higher recoil impulse (both rearward and forward), but it will also do its job more consistently in a wider range of conditions. Reliability is king in practical scenarios.

Recommendation for Suppressed Shooters

If you shoot suppressed, you want to go heavy. The addition of a suppressor will increase the dwell time and back pressure because the gas is captured in the suppressor and dissipated after the round leaves the muzzle. The excess back pressure will look for the path of least resistance, which will include back through the barrel, through the gas system, and around the spent case (as the bolt begins to extract it). As that excess gas escapes into the Upper Receiver, it will increase fouling and may give you a nice blast of hot gas in your face. The best way to counter this is to delay the movement of the reciprocating mass (via higher mass and spring weight) and regulate the gas (via an adjustable gas block). Less gas in the face and lower fouling will come at the expense of a higher recoil impulse and slower cycling.

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