enough to either prevent insertion of the cartridge into the chamber, or – with high velocity rounds – cause dangerously high pressures in the barrel, potentially rupturing the cartridge case and injuring the shooter; for that reason, standard and subsonic cartridges usually use a wax lubricant on lead bullets. Variants The variety of .22 LR loads are often divided into four distinct categories, based on nominal velocity: Subsonic-velocity: which also includes "target" or "match" loads, at nominal speeds below . Standard-velocity: High-velocity: Hyper-velocity, or Ultra-velocity: over Subsonic Subsonic rounds have a muzzle velocity of less than the speed of sound (about ). These rounds are sometimes equipped with extra-heavy bullets of to improve the terminal ballistics of the slower projectile. Conversely, these rounds may contain little more than a primer and an extra-light bullet. Subsonic rounds are favored by some shooters due to slightly superior accuracy and reduction in noise. Supersonic rounds produce a loud crack which can scare away animals when hunting. Accuracy is reportedly improved with subsonic rounds because a supersonic bullet (or projectile) that slows from supersonic to subsonic speed undergoes drastic aerodynamic changes in this transonic zone that might adversely affect the stability and accuracy of the bullet. Because the speed of sound in air at is about , the subsonic round's muzzle velocity is slightly below the speed of sound under many hunting conditions. However, under cold air conditions at , the speed of sound drops to , approximately muzzle velocity. Hence, a "subsonic" round used below this temperature may be supersonic, and during the transition from supersonic to subsonic velocity, it may become unstable, reducing accuracy. To counteract this, some cartridge manufacturers have lowered the speed of their subsonic ammunition to or less. Various combinations of subsonic rounds and semiautomatic .22 LR firearms result in unreliable cycling of the firearms' actions, as the result of insufficient recoil energy. Some subsonic rounds use heavier bullets (achieving lower velocities) to ensure, as a result of increased bullet mass, that enough energy is produced to cycle common blow-back actions. As an example, the Aguila .22 LR SSS "Sniper SubSonic" round, has a bullet on a .22 short case, providing the cartridge the same overall dimensions as a .22 long rifle round. However, other problems may be encountered: the heavier and longer bullet of the Aguila cartridge requires a faster barrel twist (by the Greenhill formula) to ensure