A thorny topic plaguing the modern military assault rifle world in the guise of 5.56mm versus 7.62mm round also has its equivalent in the Airsoft world – i.e. 0.2-gram 6mm BB pellets versus heavier types. Can the laws of physics offer insight?

By: Ringo Bones

Ever since the debate on which weapon system is best – automatic assault rifles that fire the 5.56mm X 45mm cartridge versus ones designed to fire the 7.62mm X 51mm NATO round – arose during the Vietnam War. Many a servicemen and gun enthusiast around the world readily embraced the resultant maxim of the debate. A maxim stating that the heavier the bullet, the more accurate and energetic it is – i.e. more kinetic energy. More kinetic energy equals more killing power. Thus, making everyone and his dog in the Airsoft AEG weapons system gaming community to stock-up on heavier 0.25-gram 6mm plastic BB pellets – even heavier 0.3 and 0.5-gram types – in lieu of the standard / regulation-approved 0.2-gram 6mm plastic BB pellets. The question now is, are their “obsession” with heavier plastic BB pellets grounded in the laws of everyday Newtonian physics?

Just because Airsoft AEG 6mm plastic BB pellet-firing weapons systems produce even lower recoil than the latest super V recoil vectoring technology-equipped KRIS submachine gun doesn’t mean that Airsoft enthusiast are entitled – even wise - to use the heaviest 6mm plastic BB pellets available. Apart from engendering a 0.2-gram versus 0.25-gram or even a 0.3-gram debate in the Airsoft world, and here’s the reason why.

The typical formula for calculating the kinetic energy of a typical projectile – i.e. how hard a bullet will hit the target, is typically calculated using the formula Ke=½m•v². Where: Ke or kinetic energy is equal to one-half of the mass (m) of the projectile or bullet multiplied by the square of the velocity (v²) or velocity multiplied by the velocity of the projectile. For calculating convenience, we’ll use meters per second for velocity, even though a typical chronograph (the device used for measuring the muzzle velocity in an Airsoft competition) is set to measure the velocity in feet per second. Also, don’t forget to convert the given weight of your 6mm BB pellets weight into kilograms (example 0.2-gram BB = 0.0002Kg). This is necessary because the kinetic energy formula Ke=½m•v² is an MKS (meter, kilogram, second) formula which gives the answer in Joules or kilogram meter per second, as a measure of kinetic energy.

Example: given that a certain Airsoft AEG weapons system fired an 0.2-gram BB pellet at a muzzle velocity of 128 meters per second (about 420 feet per second) about the same muzzle velocity as my own Airsoft AEG gun when measured from a competition-sanctioned chronograph. This works out to be: Ke=½ (0.0002kg)•(128 meters / second)² = 1.6384 Joules, which is about the same energy rating as the spring driving the piston primarily responsible for throwing that 0.2-gram 6mm plastic BB pellet to it’s intended target at such velocity. Since the energy transfer in the gearbox and piston system of a typical Airsoft AEG weapons system is conserved – i.e. you can’t pack in more nitrocellulose propellant – a heavier 6mm BB pellet will only result in a velocity slowdown. This is so because the spring can only transfer 1.6384 Joules worth of kinetic energy. In my Airsoft AEG gun, using a 0.25-gram 6mm BB pellet caused the chronograph reading to fall to 114 meters per second (374 feet per second) in accordance with the law of energy conservation. Using lighter 0.12-gram 6mm BB pellets allowed my Airsoft gun to fire at a muzzle velocity of 165 meters per second (541meters per second), but this resulted in a wildly inaccurate trajectory since 0.12-gram BB s are just too light given their inferior ballistic coefficient characteristics. The mathematical / theoretical results might be sobering, but how is it in practice?

My first-hand experience points out that heavier BB pellets suffer less horizontal deflection in flight, especially on windy conditions, but your range can be compromised by as much as 20%. About the same as that resulting from using a silencer / suppressor on a typical sniper rifle. Also, the energy transfer of the pellet is about the same as it’s lighter counterpart since you are only using the same spring with the same energy transfer characteristics.

As a “mere toy”, Airsoft AEG has surprisingly gained widespread acceptance among “strike teams” around the world for their CQB / room clearing training sessions because they are much safer than real guns being loaded with paintball-type ammunition being shot at their fellow team mates. Despite it’s inherent range limitations due to the woefully poor ballistic coefficient characteristics of your typical 6mm plastic BB pellets, these various government and law-enforcement strike teams have learned to live with the Airsoft AEG gun’s limitations. Shouldn’t those “mere civilians / pseudo militias” Airsoft enthusiasts learn to accept them too?

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