Tuning the AR Operating System
At this point, we have discussed the way the gas system works and the importance of putting the right amount of pressure and flow through the gas system. We’ve talked about the opposing forces of the gas system and the buffering system. We know what the reciprocating mass does and how it does it. Everything about the way the gas gun operates centers around the gas.
The direct impingement AR is all about force. That force is harness by the gas system and is counteracted by the buffering system. This system relies on balance.
- You need to make sure you have enough gas to run the system — reliably. This means ensuring an efficient gas system — keep and use the gas you have.
- You need to avoid too much gas. This means managing the flow of gas into and through the system.
- You need manage the force delivered by the gas system. We do this by countering the force with the buffering system.
Option 1: Build It Right
The best way to fix your problems with your build is to build it right the first time. Our rules for a reliable build are as follows:
- Ensure an efficient gas system. This is the most important thing you can focus on. You need tight junctions from the barrel to the gas rings. We’ve discussed this in our Gas System Efficiency article and humbly offer our specifications in our PBA Specs article. If you have an efficient system, you will probably have an over-gassed gun (which is good).
- Use a full mass bolt carrier. If you have an efficient gas system, you will rarely need anything lighter than a full mass carrier. The higher inertia of a full mass carrier give you the momentum needed to push through inconsistencies and obstructions.
- Use the longest functional gas system for your barrel length. You want to delay the pressurization as long as possible, without venturing into insufficient dwell time. The longer you can delay the pressurization, the lower the system pressures, bolt thrust, and wasted gas.
- Install an adjustable gas block. If you have an efficient gas system, you should be over-gassed. Modulating gas flow into the system with an adjustable gas block is the best way to ensure that your system will deliver optimal performance and can adapt to any conditions.
- Use the appropriate combination of buffer and buffer spring. Assuming you’ve done everything else right, standard combinations of buffer and spring should work. If not, this is the time to find the right combination. Do NOT start here.
Option 2: Fix Your Build
If you find yourself encountering any of the following symptoms, you probably already have a problem gun and it may have something to do with the operating system:
- Failure-to-Fire
- Failure-to-Extract
- Failure-to-Eject
- Failure-to-Feed
- Bolt Catch Failure
- Short Cycling
- Excessive Recoil
- Mushroomed Buffer Bumper
- Gas Key Hitting Lower
- Over-gassed Ejection Pattern
- Under-gassed Ejection Pattern
- Dented Brass
- Popped Primers
- Broken Bolt Lugs
- Broken Bolt Body
We will approach the tuning of the AR from the perspective of the gas system and buffering system. The fire control group and barrel (unless out of spec) are generally not part of the tuning process.
Too Gassy (easy to fix)
Too much gas (or too little buffering) is easy to deal with. There are several ways to control excess gas:
- Deal with the excessive force generated by the gas with inertia (heavier buffer and carrier)
- Deal with the excessive force generated by the gas with spring tension (buffer spring).
- Deal with excess gas at the gas key (adjustable gas key; we will not discuss these further).
- Deal with the excess gas at the gas block (adjustable gas block).
Most of these options are treating the symptoms of excess gas entering the system. The longer you wait to deal with the gas, the more ill effects it can have (and the more problems you create with the “solutions”).
Carrier Mass
We always recommend running full mass bolt carrier groups. A full mass carrier will always be the most reliable option. If your gun is over-gassed with a light carrier, swap it out for a full mass carrier; this could fix your issue.
In a standard caliber AR-15, light carriers are only good for one thing: finely tuned competition race guns that require every millisecond of advantage. These guns are always pristine, impeccably lubricated, and run consistently manufactured ammunition loaded into perfect magazines. However, when the ammo changes, the temperature changes, the gun gets dirty, or any other variable is introduced, guns with a light carrier are much more prone to malfunction. This is because their reciprocating mass (BCG and buffer) have lower inertia and are more sensitive to variation.
One could argue that a lightweight bolt carrier is also good for certain cartridges, like sub-sonic .300 AAC Blackout. But this is not the approach we would take.
Buffer Mass
When your gun is over-gassed, the force generated by the gas system is more than the operating system needs to cycle. Inertia resists this force: a heavier reciprocating mass (BCG and/or buffer) will require more energy to get it moving.
Not every AR likes the same buffer weight. Unabated, each gas system length generates different levels of force to the BCG. You can manage these differences in force by swapping out your buffer to a different weight. The following table reflects the typical buffer weight that works well in each gas system length using a standard spring for the buffer system length (i.e. carbine spring for carbine receiver extension; rifle spring for rifle receiver extension).
If your gun is over-gassed (and showing symptoms of being significantly over-gassed), try switching to a heavier buffer. Experiment with different buffer weights and buffer spring combinations.
The table below offers a general guideline for buffer selection. However, remember that there are other variables that need to be considered, including cartridge load/pressure (e.g. sub-sonic versus standard versus hot loads), use of a suppressor, gas port size, and gas system efficiency.
Buffer Spring Strength
The buffer spring absorbs, stores, and releases the energy transferred to the reciprocating mass by the gas system. A stronger spring will absorb and release more energy than a lighter spring.
Springs wear out. Your typical round wire buffer spring will wear out faster than you would think (some say as few as 500 rounds will noticeably affect the spring). As the spring wears out, it shortens and the tension applied to the reciprocating mass is reduced. This causes the mass to cycle harder and faster, which can result in malfunctions. As a rule of thumb, a typical round wire spring should be replaced once the uncompressed length shrinks below 10 1/16” (carbine) or 11 ¾” (rifle).
When you increase the inertia of the reciprocating mass (i.e. full mass carrier and heavy buffer), you should also consider the strength of the buffer spring. A heavier mass moving at the same speed carries more kinetic energy. The spring needs to be able to absorb this added energy, or the mass will simply slam into the back of the buffer tube.
Along with the buffer weight, playing with heavier spring weights can help you tune an over-gassed gun.
Sprinco makes a full line of buffer springs that can be used to tame any level of recoil. And the manufacturing process that Sprinco uses for all of their buffer springs results in a stronger, more durable, and longer-lasting spring.
David Tubb makes the best flat wire spring on the market. The properties of a flat wire spring offer higher bolt tension and a more linear (consistent) tension curve. They also last significantly longer than round wire springs.
Adjustable Gas Block
The best place to deal with excess gas is at the source. Assuming you aren’t drilling your own gas port, the next best place to deal with excess gas is the gas block. Before we get into adjustable gas blocks, it is worth understanding the what a fixed is.
Fixed Gas Block
All gas blocks are basically a hunk of metal that work to direct the flow of gas into the gas tube and turn it 90 degrees to the rear. That’s pretty much all a fixed gas block does.
Adjustable Gas Block
Adjustable (or tunable) gas blocks add a component of modulation to a fixed gas block. They work on one of two principles, both or which are effective mechanisms for modulating excess gas pressure. By restricting or bleeding off the gas pressure at the gas block, you can send just enough gas to through the gas system to cycle the gun. And if things change (change in ambient temperature, switching to lower/higher pressure ammo, worn parts, a dirty gun, under-lubrication, etc.), you can always dial the gas flow up or down.
Benefits
Simpler design. Fewer parts to fail.
Drawbacks
Restriction screw is susceptible to erosion and seizing.
Depending on the design, the detent mechanism is often susceptible to fouling and lockup of the adjustment screw or seizing of the detent.
Double set screws can be a challenge to lock into the right setting, because the second set screw often rotates the first out of the adjusted position.
In a bleed off gas block, the adjustment mechanism is an incrementally adjustable system allows the venting of gas. This mechanism does not restrict the flow of gas into the gas system; instead, it diverts a portion of the gas through the vent, effectively reducing the system pressure.
We are only aware of one manufacturer of a bleed-off gas block: Superlative Arms. Their DI Adjustable Gas Block is an amazing feat of engineering. The device can be set to either restrictive or bleed-off modes. We’ve covered the restrictive mechanism but will add that SupArms’ external detent design prevents fouling and lockup of the detent mechanism. The bleed-off mode of the device vents excess gas toward the muzzle of the gun through a channel that runs parallel to the adjustment screw.
Benefits
Lower recoil
Less heat
Less fouling
Great for suppressed
Drawbacks
More expensive than restrictive blocks
Not Gassy Enough (hard to fix)
Dealing with too little gas (if not caused by overt over-buffering) can be a lot harder to cope with. There are a couple of tactics that you can use.
- Make sure that you can send enough gas into the system (i.e. the gas port is the right size)
- Make sure that you are sending enough gas into the system (i.e. make sure the gas port and gas block are aligned, etc.).
- Reduce the inertia and spring tension.
- Keep the gas in the gas system by eliminating leaks in the system.
Most gas ports are appropriately sized (or even oversized), so you generally don’t have to worry about #1.
If your gun is under-gassed, gas block misalignment is a real possibility. If the gas block is too far forward or rearward, or is rotated on the barrel, the edges of the hole in the gas block mat block off some of the gas port, effectively reducing the size of the orifice. This will reduce the amount of gas that can get into your gas system. For guidance on correct installation of the gas block, please check out Build Instruction Step U4.
In a standard caliber gun, if you are sending enough gas into the gas system and you are keeping it there, you should never need to play around with lighter BCGs, lighter buffers, and lighter springs. When you start playing around with lighter components, you will sacrifice reliability and durability. As such, we recommend option #3 as a last resort or for lighter loads (e.g. 300 AAC Blackout sub-sonic).
Unfortunately, most gas systems leak like a sieve and could use a bit of attention to tighten them up. The only way to address this is by focusing on dimensional tolerances of the various components of the gas system as you are building/re-building. We will explore tolerance in the next article.
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