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The buffer or action spring makes self-loading operation possible. In an AR-15, there are several buffer springs lengths and designs to choose from, each with its unique characteristics or advantages.
What Does a Buffer Spring Do?
The buffer assembly in an AR-15 consists of the buffer and the buffer spring. The buffer spring is also called the action spring, recoil spring, or return spring. Regardless of how a semi-automatic firearm operates — by gas, recoil, or blowback — the return spring is necessary to cycling.
To understand the role that the buffer spring plays in the AR-15 platform, it’s important to understand how the AR-15 operates. The cycle of operation in a self-loading firearm comprises the following eight functions:
The AR-15 is a semi-automatic, gas-operated firearm fed from a detachable box magazine. When you press the trigger, the hammer is driven forward by the hammer spring, which sends the firing pin forward through a hole in the breech face and into the cartridge primer.
When the primer detonates, it ignites the propellant charge in the cartridge. As the propellant burns, it generates high-pressure expanding gases. In addition to propelling the bullet through the barrel, these gases are also the power source behind the rifle’s self-loading operation.
Gases pass through a port in the barrel and are fed through a tube into the bolt-carrier group. There’s an expansion chamber formed between the bolt and bolt carrier. When the gases enter this chamber, they apply pressure against the bolt carrier, forcing it rearward.
The bolt carrier moves to the rear, cams the bolt open, unlocking and withdrawing it from the barrel extension. On the rearward stroke, the tail of the bolt carrier hits the buffer and compresses the buffer spring.
The buffer adds weight to the bolt carrier, and the spring adds resistance. Both serve to decelerate the bolt carrier group.
As the buffer spring expands, it returns the bolt carrier group to its forwardmost position, stripping a round from the magazine (if there is ammunition remaining), feeding it into the chamber, and camming the bolt closed. This locks the bolt to the barrel extension, readying the weapon for firing.
AR-15 Buffer Spring Differences
The AR-15 has a buffer tube or receiver extension attached to the lower receiver. This serves as the housing for the buffer assembly, which includes the spring.
The buffer spring encircles the buffer, which is retained by a pin. When the AR-15 cycles, the spring compresses and expands inside the receiver extension.
There are a variety of different buffer springs available, each with distinct advantages or specialized applications. When you’re learning about AR-15 buffer spring differences, it’s important to consider the length that you need.
Buffer Spring Length
The buffer you need for your AR-15 depends on many factors, from the barrel length and position of the gas port to the cartridge you intend to fire. For example, a heavier buffer will be necessary when using subsonic .300 Blackout ammunition and the more powerful .458 SOCOM and .50 Beowulf cartridges.
Carbine-length buffers differ from rifle-length buffers, and the buffer springs are also different. A standard carbine buffer spring is 10.5” in length and has 37–39 coils.
However, a standard rifle buffer spring has a length of 12.5” and 41–42 coils. These buffer springs are not interchangeable — they are intended for different-length receiver extensions and buffers. You should only select the buffer assembly that is appropriate for your weapon.
Spring Set and Damage
When a coil spring compresses and expands repeatedly, it will eventually shorten. This is called “spring set.” It can take thousands of rounds of ammunition to cause the buffer spring to set.
If you fire high round counts through your AR-15 due to your participation in competitive shooting sports or tactical training courses, you should periodically evaluate your buffer spring. You can compare it against a new spring to determine whether it has lost length. It’s also advisable to inspect the buffer for wear. When a buffer becomes worn out, the rubber part that hits the end of the receiver extension may begin to expand or spread.
The shorter the buffer spring becomes, the less force or load it can generate. This can adversely affect the cycle, either causing the weapon to open too quickly or close with insufficient velocity to chamber and lock.
As long as the carbine and rifle buffer springs measure more than 10 and 12” respectively, you may still expect reasonable functionality. However, once your buffer spring shortens to a length less than these, you should discard and replace it. Likewise, if you notice that it’s bent or is showing signs of corrosion, install a new one.
Round vs. Flat Wire
Buffer springs are available in both round- and flat-wire designs. The advantage of a flat-wire spring is that the flat-wire spring will be shorter for the same number of coils. This allows you to use a spring capable of increased compression and expansion.
Many shooters find that flat-wire springs also reduce the twang associated with spring compression and expansion inside the receiver extension.
Another relative newcomer to the buffer spring market is the braided-wire spring. The German MG 42 recoil-operated, roller-locked machine gun uses a braided-wire recoil spring, which is where modern manufacturers drew inspiration.
Each coil in a braided-wire buffer spring consists of three separate wire strands. The braided design increases the strength of the spring, allows it to absorb more energy, and increases its service life. It may also serve to dampen the vibration during the cycle.
The material that you choose is also important to the functionality of your AR-15. Stainless-steel buffer springs will resist corrosion more effectively — and corroded springs are a liability.
Increased corrosion resistance may not be critical if you live in an arid environment, such as the American southwest. However, in highly humid climates and during heavy rain and snowfall, it can be an advantage.
Noise or Twang
Depending on the configuration of your AR-15, you may notice what’s often called a twang. This is noise caused by the spring reciprocating and contacting the inside the receiver extension. This doesn’t only cause noise — it also causes vibration and roughness.
If you want to minimize or eliminate this phenomenon, there are a few options available. The aforementioned braided design dampens the vibration, which reduces noise. You may also select buffer springs with special coatings or platings to minimize friction against the inside of the extension.
If your OEM spring exhibits this kind of noise, it’s important to remember that it doesn’t indicate a functional deficit or reduced service life.
Aftermarket springs can reduce or virtually eliminate this effect, increasing the smoothness of the action. However, it does not impede reliable cycling. Among AR-15 buffer spring differences, this is one of the most immediately noticeable.
The AR-15 accessories and spare parts market offers a wide variety of different buffer springs and assemblies for the tinkerer, hobbyist, or gunsmith. They can increase the service life, durability, and smoothness of your weapon’s operation. Find out which kind of buffer spring you need, and don’t be afraid to upgrade your rifle.