Airsoft, like any other sport, involves many scientific principles that go relatively unnoticed. In the heat of battle, one rarely worries about something like the transfer of kinetic energy. While knowing airsoft physics will not realistically help you in a game, the principles are good to know. Knowledge of pellet ballistics, on the other hand, can help you with your game.
Pellets transfer very small amounts of kinetic energy mainly because of their size. The transfer of energy is basically the power of impact. Airsoft and paintball energy transfers have been compared, and paintball transfers substantially more energy. Energy transfers are measured in joules, the SI derived unit of energy. A standard .20g BB traveling at 300ft/s transfers .8 joules, while a standard paintball traveling at the same speed transfers almost 12 joules. Energy transfer can actually be calculated using a mathematical formula; E = 1/2mv2, where E is joules, m is mass in kilograms, and v is velocity in meters per second. Since paintballs transfer more kinetic energy, they could be considered more damaging than airsoft pellets.
The weight of airsoft pellets significantly affects their speed and trajectory. The lighter a pellet is, the faster it will travel, but it will also be less accurate due to the fact it will be more susceptible to environmental factors like wind. Heavier pellets have straighter trajectories, so they are favorable to lighter pellets, but in many cases heavier pellets cannot be used because the airsoft gun utilizing them is not strong enough to propel the pellets at a fast rate. Therefore, players that wish to use pellets heavier than .25g usually need to upgrade their gun. Another benefit of heavy pellets is that they decelerate slowly, unlike light pellets, which start fast but quickly lose speed. Airsoft snipers usually use .30g pellets because of their high stability, but sniper weights can reach as high as .43g (although very expensive upgrades are required to use this grade of pellet). The heaviest pellet is .88g, which is never used in airsoft because it is incredibly slow, not to mention very dangerous, as it is usually made of steel.
Pellet velocity is determined primarily by the tension of the spring being used to propel the pellet, or in the case of gas airsoft guns, the type of gas being used. Energy transfer is relative to the speed of the pellet, and speed is also heavily affected by weight. For example, a .12g pellet using .800J (the common energy rate of airsoft) of energy will initially travel 375fps. However, a .20g pellet utilizing the same amount of energy only travels about 280fps, and a .45g pellet will travel at about 200fps. For a .45g pellet to reach the velocity of a 375fps .12g pellet, it would need to utilize more than 2.50J of energy.
Bernoulli’s principle is the method through which pellets fly. It is, interestingly, also the principle through which airplanes attain lift. Bernoulli’s principle states that the velocities of fluids increase with a decrease in pressure. In relation to airsoft, there is a fluid (air) above the pellet at a relatively high velocity, which means the pressure above the airsoft pellet is lower than the pressure under it, as there is a smaller amount of air flowing under the pellet as there is above it. Therefore, the pressure under the airsoft pellet pushes up and lifts the pellet, allowing it to fly for a long period of time. Hop-up systems in airsoft guns apply backspin to pellets, making air travel faster along the top of the pellet and creating a larger difference in pressure between the top and bottom.