Rimfire ammunition

Rimfire ammunition, from left to right, .22 Short, .22 Long Rifle, .22 WMR, .17 HM2, .17 HMR

A rimfire is a type of firearm cartridge. It is called a rimfire because instead of the firing pin striking the primer cap at the center of the base of the cartridge to ignite it (as in a centerfire cartridge), the pin strikes the base's rim. The rim of the rimfire cartridge is essentially an extended and widened percussion cap which contains the priming compound, while the cartridge case itself contains the propellant powder and the projectile (bullet). Once the rim of cartridge has been struck and the bullet discharged, the cartridge cannot be reloaded, because the head has been deformed by the firing pin impact. While many different cartridge priming methods have been tried, only rimfire and centerfire survive today in significant use.

Characteristics

Rimfire cartridges are limited to low pressure calibers due to the thin case they require for the firing pin to be able to crush the rim and ignite the primer. Although rimfire calibers up to .44 (11 mm) were once common, modern rimfires tend to be of caliber .22 (5.5 mm) or smaller. The low pressures mean that rimfire firearms can be very light and inexpensive, which has helped lead to the continuing popularity of small caliber rimfire cartridges.

Economics

Rimfire cartridges are typically inexpensive, primarily due to the inherent cost-efficiency of the ability to make large production quantities (called "lots"). Until 2002, a "brick" of 500 inexpensive .22 Long Rifle cartridges typically cost less than US$10.00. Beginning in 2003, the price of metals used in cartridges (lead, copper and zinc) increased dramatically. This caused the typical price of a "brick" of .22 long rifle cartridges to increase to $15.00 to $20.00, with single boxes of 50 rounds going for about $1.75 to $2.50 each. Premium or match-grade .22 Long Rifle cartridges, as well as less common or out-of-production rimfire cartridges (such as the .22 Short, .22 Long, .22 Extra Long, .22 Winchester Auto, and 5 mm Remington Rimfire Magnum) can cost substantially more.

History

The idea of placing a priming compound in the rim of the cartridge evolved from an 1831 patent, which called for a thin case, coated all along the inside with priming compound. By 1845, this had evolved into the Flobert .22 BB Cap, which distributed the priming compound just inside the rim. The BB Cap, intended for use in an indoor "gallery" target rifle, used no gunpowder, but relied entirely on the priming compound for propulsion. Dating back to 1845, the .22 BB Cap is essentially just a percussion cap with a round ball pressed in the front, and a rim to hold it securely in the chamber. Velocities are very low, comparable to an airgun, as the round was intended for use in indoor shooting galleries. The next rimfire cartridge was the .22 Short, developed for Smith and Wesson's first revolver in 1857; it used a longer rimfire case and 4 grains (260 mg) of black powder to fire a conical bullet. This led to the .22 Long, with a longer case and 5 grains (320 mg) of black powder. The .22 Long Rifle is a .22 Long case loaded with a longer, heavier bullet intended for better performance in the long barrel of a rifle. The .22 Long Rifle is the most common cartridge in the world. While larger rimfire calibers were made, such as the .41 Rimfire Short, the .44 Henry Flat devised for the famous Henry Repeating Rifle, up to the .58 Miller, the larger calibers were quickly replaced by centerfire versions, and today the .22 caliber rimfires are all that survive of the early rimfires. The early 21st century has seen a revival in interest in rimfire cartridges, with two new rimfires introduced, both in .17 caliber (4.5 mm).

Below is a list of the most common current production rimfire ammunition:

• The powderless .22 Cap rounds, including BB Cap.
• .22 Short, used for target shooting and Olympic and ISSF 25 m Rapid Fire Pistol competition until 2005
• .22 Long (obsolete)
• .22 Long Rifle (.22 LR), the most common cartridge made
• .22 Long Rifle High Velocity Mini-Mag (.22 LR)
• .22 Stinger (slightly longer case, same overall loaded length) - the basis for the .17 HM2
• .22 Extra Long (Obsolete)
• .22 Winchester Rimfire (.22 WRF) AKA .22 Remington Special (obsolete)
• .22 Winchester Magnum Rimfire (.22 WMR)
• .17 Hornady Magnum Rimfire (.17 HMR), a .17 caliber based on the .22 WMR case
• .17 Hornady Mach 2 (.17 HM2), a .17 caliber based on the .22 Stinger case
• 5 mm Remington Rimfire Magnum (recently put back into production by Aguila Ammunition/Centurion)

A new and increasingly popular rimfire, the 17 HMR is basically a .22 WMR with a smaller formed neck which accepts a .17 bullet. The advantages of the 17 HMR over .22 WMR and other rimfires are its much flatter trajectory, its highly frangible hollow point bullets (often with plastic "ballistic tips" that improve the external ballistics performance), and the lower retail cost relative to WMR. The .17 HM2 [Hornardy Mach 2] is based on the .22 Long Rifle and offers similar performance advantages over its parent cartridge, at a significantly higher cost. While .17 HM2 sells for about six times the cost of .22 Long Rifle ammunition (per box of 50 rounds), it is still significantly cheaper than most centerfire ammunition, and somewhat cheaper than the .17 HMR.

A notable rimfire still in production in Europe, and chambered by the Winchester Model 39 in the 1920s, is the 9 mm Flobert. This cartridge can fire a small ball, but is primarily loaded with a small amount of shot, and used in smoothbore guns as a miniature shotgun, or "garden gun". Power and range are very limited, making it suitable only for pest control.

Shot

There is a type of ammunition for the .22 LR that fires a small amount of #11 or #12 shot (about 1/15th ounce). The shot is only marginally effective in close ranges, and is usually used for shooting rats or other small animals. At a distance of about 10 feet (3 m) the pattern is about 8 inches (20 cm) in diameter from a standard rifle, which is about the maximum effective range. Special smoothbore shotguns, such as Marlin's Garden Gun can produce effective patterns out to 15 or 20 yards using .22 WMR shotshells, which hold 1/8 oz. of #11 or #12 shot contained in a plastic capsule.

Shotshells will not feed reliably in some magazine fed firearms, due to the unusual shape of crimped brands, and the relatively fragile plastic tips of other brands. Shotshells will also not produce sufficient power to cycle semiautomatic actions, because, unlike projectile ammunition, nothing forms to the lands and grooves of the barrel to create the pressure necessary to cycle the firearm's action.

Collectibility

Rimfire ammunition is popular with ammunition cartridge collectors, who base much of the collectibility value of rimfire cartridges on the rarity of the stamped mark on the head of the cartridge (the headstamp). There is a subcategory of collectible rimfire ammunition with headstamps commemorating certain persons who have worked in the industry, often issued in extremely small quantities on the occasion of that person's retirement. Often the majority of these cartridges are given to the retiring individual, leaving him or her to decide to whom they are traded or distributed. This increases their perceived value as collectibles.

References

• Marlin's "Garden Gun" - Model 25MG Guns Magazine, March, 2000 by Clair Rees