The .22 Long Rifle (22lr) (metric designation: 5.6×15mmR) cartridge is a long-established variety of .22 caliber rimfire ammunition, and in terms of units sold the .22 Long Rifle (22lr) is still by far the most common ammunition in the world today.

The .22 Long Rifle (22lr) cartridge is often referred to simply as .22 LR and various rifles, pistols, revolvers, submachine guns and even some smoothbore shotguns (No. 1 bore) have been manufactured in this caliber.

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American firearms manufacturer J. Stevens Arms & Tool Company introduced the cartridge in 1887 by combining the casing of the .22 Long with the 40-grain (2.6 g) bullet of the .22 Extra Long. The round owes its origin to the Flobert BB cap of 1845 through the .22 Smith & Wesson cartridge of 1857. It is one of the few cartridges that are accepted by a large variety of rifles, as well as pistols. 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.

Low cost, minimal recoil, and relatively low noise make the .22 LR an ideal cartridge for recreational shooting, initial firearms training, small-game hunting, and pest control. Used by Boy Scouts in the United States for the rifle shooting merit badge, the cartridge is popular among novice shooters and experts alike. The rimfire round is commonly packaged in boxes of 50 or 100 rounds, and is often sold by the ‘brick’, a carton containing either 10 boxes of 50 rounds or loose cartridges totalling 500 rounds, or the ‘case’ containing 10 bricks totalling 5,000 rounds. .22 LR ammunition is among the least costly cartridge ammunition available.

A wide variety of rimfire ammunition is available commercially, and the available ammunition varies widely both in price and performance. Bullet weights among commercially available ammunition range from 20 to 60 grains (1.3 to 3.9 g), and velocities vary from 575 to 1,750 ft/s (175 to 533 m/s). Promotional loads for plinking can be purchased in bulk for significantly less cost than precision target rounds. The low cost of ammunition has a substantial effect on the popularity of the .22 LR. For this reason, rimfire cartridges are commonly used for target practice.

The low recoil of the cartridge makes it ideal for introductory firearms courses. Novice shooters can be surprised or frightened by the recoil of more powerful rounds. Beginners shooting firearms beyond their comfort level frequently develop a habit of flinching in an attempt to counter anticipated recoil. The resulting habit impedes correct posture and follow-through at the most critical phase of the shot and is difficult to correct. With high recoil eliminated, other errors in marksmanship technique are easier to identify and correct.

Available for this round are AR-15 upper receivers and M1911 slide assemblies. Many handgun manufacturers have an upper pistol conversion kit to make it shoot .22 ammunition. These rimfire conversions allow shooters to practice inexpensively while retaining the handling characteristics of their chosen firearms (with reduced recoil and muzzle blast). Additionally, .22 conversion kits allow practice at indoor ranges which prohibit high-power firearms. Owners of guns that use gas systems, such as AR-15 sport style rifles, normally avoid firing non-jacketed .22 LR ammunition, as the use of unjacketed ammunition may lead to lead fouling of the gas port inside the barrel and costly gunsmithing procedures.

Annual production is estimated by some at 2–2.5 billion rounds. The NSSF estimates that half of the US production of 10 billion cartridges is composed of .22LR.[9] Despite the high production figures there have occasionally been shortages of .22 LR in the continental United States,