by muzzle energy as well as barrel length and ammunition diameter. Some jurisdictions also stipulate minimum muzzle energies for safe hunting. For example, in Denmark rifle ammunition used for hunting the largest types of game there such as red deer must have a kinetic energy E100 (i.e.: at 100 m range) of at least 2700 J and a bullet mass of at least 9 g or alternatively an E100 of at least 2000 J and a bullet mass of at least 10 g. Namibia specifies three levels of minimum muzzle energy for hunting, 1350 J for game such as springbok, 2700 J for game such as hartebeest and 5400 J for big game, together with a minimum caliber of 7 mm. In Germany airsoft guns with a muzzle energy of no more than 0.5 J are exempt from the gun law, while air guns with a muzzle energy of no more than 7.5 J may be acquired without a firearms license. Mainland China uses a varied concept of "muzzle ratio kinetic energy" (), which is the quotient (ratio) of the muzzle energy divided by the bore cross sectional area, to distinguish genuine guns from "imitation" replicas like toy guns. The Ministry of Public Security unilaterally introduced the concept in 2008 leading up to the Beijing Olympic Games, dictating that anything over 1.8 J/cm2 to be defined as real firearms. This caused many existing toy gun products on the Chinese market (particularly airsoft) to become illegal overnight, as almost all airsoft guns shooting a standard 0.20 gram 6 mm pellet have a muzzle velocity over , which translates to more than of muzzle energy, or 2.0536 J/cm2 of "ratio energy". For comparison a standard baseball changeup thrown at has 1.951 J/cm2 of "ratio energy" which also exceeds the 1.8 J/cm2 level of a real firearm while a fastball can reach over 3.5 J/cm2 or nearly double the level of a real firearm. The subsequent crackdowns by local law enforcement led to many seizures, arrests and prosecutions of individual owners for "trafficking and possession of illegal weapons" over the years for weapons that were previously permitted. See also Free recoil Muzzle velocity Power factor (shooting sports) Resources Edward F. Obert, Thermodynamics, McGraw-Hill Book Co., 1948. Mc Graw-Hill encyclopedia of Science and Technology, volume ebe-eye and ice-lev, 9th Edition, Mc Graw-Hill, 2002. References Ammunition Ballistics