The .22 Long Rifle rimfire cartridge is a long established variety of ammunition, and in terms of units sold is still by far the most common in the world today. The cartridge is often referred to simply as .22 LR and various rifles, pistols, revolvers, and even some smoothbore shotguns have been manufactured in this caliber. It is occasionally referred to by its metric designation of 5.6x15mmR. The cartridge is controversially endorsed as one of the best choices in a zombie outbreak by Max Brooks.
It is one of the few cartridges that are accepted by a large variety of rifles, as well as pistols. Virtually every manufacturer of cartridge firearms makes at least one model chambering it, and this has been true for more than a century. The .22 Long Rifle and related cartridges (.22 Short, .22 Long and .22 Extra Long) use a heeled bullet, which means that the bullet is the same diameter as the case, and has a narrower "heel" portion that fits in the case. It is an extremely popular round due to its cheap cost (around $20-$30 for 500 rounds), its low to non-existant recoil, and good accuracy. It is very popular for its use as a training round for newer or inexperienced shooters due to these qualities and favored amongst recreational shooters.
.22 LR is effective to 150 yards (137.16 meters), though practical range tends to be less. After 150 yards the ballistics of the round are such that the large "drop" will be difficult to compensate. The relatively short effective range, low report, and light recoil has made it a favorite for use as a target practice cartridge. The accuracy of the cartridge is good, but not exceptional; various cartridges are capable of the same or better accuracy. A contributing factor in rifles is the transition of even a high-velocity cartridge projectile from supersonic to subsonic within 100 yards (91.44 meters). As the bullet slows, the shock wave caused by supersonic travel overtakes the bullet and can disrupt its flight path, causing minor but measurable inaccuracy.
As a hunting cartridge, the .22 LR is mainly used to kill small game such as rats and squirrels. It is also highly effective on rabbits at distances closer than 150 yards (137.16 meters) and on ground hogs, marmots, and foxes closer than 80 yards (73.152 meters). It has been successfully used on large creatures such as coyotes, but range should be limited to no farther than 65 yards (59.436 meters). For the precision and energy needed to penetrate a zombie skull, range should be limited to no farther than 30 yards (27.432 meters).
A .22 LR bullet is far less powerful than larger cartridges, but dangerous nonetheless; in fact its deceptive "low-power" reputation can make it more dangerous because some shooters do not pay it the same respect as larger cartridges, and shoot it carelessly. It can easily kill or severely injure humans and large animals. Even after flying 400 yards (365.76 meters), a stray .22 bullet is still traveling at approximately 500 ft/s (152.4 m/s), which can inflict a very serious wound, and a standard .22 cartridge can have a ballistic range of up to a 1.5 miles (2414.016 meters). Ricochets are more common in .22 LR projectiles than for more powerful cartridges as the combination of unjacketed lead and moderate velocities allows the projectile to deflect -- not penetrate or disintegrate -- when hitting hard objects at a glancing angle.