notes:

3,00 g is the typical weight for a paintball bullet. This weight is for comparison purposes only and is not used on Airsoft. Higher energy but different collisions read Elastic collisions topic for further information.
1 m/s = 3.281 ft/s
1 ft/s = 0.3048 m/s
Information from the United Kingdom Parliament - "Principles of firearms control" indicates the following:
Home Affairs Select Committe Second Report, 6th April 2000

"25. The Firearms Act 1968 defines a firearm "a lethal barrelled weapon of any description from which any shot, bullet or other missile can be discharged".[51] In this context, a "lethal weapon" means a weapon capable of firing a projectile with sufficient force to inflict more than a trivial injury, i.e. with a force sufficient to puncture the skin.[52] The force with which a firearm is able to deliver a projectile is normally expressed in terms of the kinetic energy it generates at its muzzle—the "muzzle energy". This force is normally expressed in units of foot pounds (ft/lb) or joules (J).[53]

26. The Home Office and the Forensic Science Service considers that the lowest level of muzzle energy capable of inflicting a penetrating wound is one foot pound (or about 1.35 joules): below these power levels, weapons are "incapable of penetrating even vulnerable parts of the body, such as the eye".[54] However, more recent analysis by the Forensic Science Agency for Northern Ireland has indicated that a more reasonable assessment of the minimum muzzle energy required to inflict a penetrating wound lies between 2.2 and 3.0 ft/lb (3-4 J).[55] We will deal more fully with this discrepancy at paragraphs 123 to 130 below."

"123. The power level at and above which an air weapon is considered a firearm in law is presently set at 1 ft/lb. However, we note above that the Forensic Science Agency of Northern Ireland has more recently assessed the power level at which a barrelled weapon is capable of inflicting a lethal wound as between 2.2 and 3 ft/lb, and the Secretary of State for Northern Ireland has proposed that the law relating to firearms in Northern Ireland be amended to take this into account.[201]"

(Link to full document at external links section)

Performance
For the sake of a relative uniform standard, the usual BB mass used when determining muzzle velocity is 0.2 g. Airsoft guns shoot 0.2 g BBs at velocities from 100 ft/s (30 m/s) for a low-end spring pistol, to 550 ft/s (170 m/s) and beyond for heavily-upgraded customized sniper rifles. Most non-upgraded AEGs using the Tokyo Marui system are in the middle, producing velocities from 270 to 300 ft/s (82 to 91 m/s), but upgrades to the internal components can increase the muzzle velocity significantly. Because of their low mass, these BBs have very little kinetic energy on impact compared to paintballs, ranging from 0.5 to more than 6 joules (J). A typical paintball at 300 ft/s (91 m/s) produces more than 12 J. This makes the sport arguably safer than Paintball although protective gear, especially for the eyes, is considered a requisite for safe play.

The total kinetic energy of a body (muzzle) can be considered (for non-relativistic mechanics) as the sum of the body's translational kinetic energy and its angular kinetic energy (also known as rotational energy). It's normal to find airsoft - kinetic calculations using only translational kinetic energy to simplify formula use.

Elastic collisions
An elastic collision occurs when two object collide and their natural governing laws "bounce" them off of one another. In another words, an elastic collision occurs only if there is no conversion of kinetic energy into other forms of energy. In Airsoft collisions are elastic since BB's (generally a rigid body) bounce off of their targets. Other examples: throwing a ball into a wall or billiard game ball collisions.

On the other side is Paintball were an impact is generally an inelastic collision because paintballs explode on contact.

Hop-up (High Operation Power UP)

- Bernoulli's principle
Bernoulli's principle is a physical law that says that if a bullet is given a backspin an overpressure is formed under the bullet and an underpressure is formed on the top of the bullet. These pressures affect the bullet as on an airplanes wings a lift is formed. The bullet is sucked up. If the Bernoulli principle is equal to the effect of gravitation the bullet will fly longer and straight until the bullet loses its energy and the Bernoulli principle stops working.

A significant development that has since been incorporated into almost all good quality mainstream airsoft guns has been "hop-up." This is a simple rubber piece around the chamber or rear of the barrel that is thicker on the top in order to provide a backspin on the BB as it exits. Consistent with Bernoulli's principle, this causes air above the BB to move slightly faster than the air below it, creating a measure of lift. The practical effect of this mechanism is immediately visible and quite effective. Using it can extend the range of an airsoft gun by up to 50%. Some guns feature an adjustment mechanism to increase or decrease the amount of backspin, which allows fine tuning of the lift generate to accommodate various BB masses. Hop-up also means that certain BB masses will not shoot in a relatively straight trajectory from certain guns - the hop-up produces either too much lift (causing the BB to "float" as it flies forward) or too little (causing a premature end to its flight). So in the words of a 12 yr old airsofter this means it puts a spin on the BB to change the presure on the top and bottom to create lift.