When many of us think the term “rimfire” we think of it as synonymous with .22 LR, but there are many other rimfire cartridges still used today. The easiest way to remember the difference between rimfire and centerfire is how the cartridge is ignited. For a centerfire cartridge, the firing pin must hit the primer in the center to ignite the powder charge while rimfire requires the outer rim of the casing to be struck for ignition. You can quickly tell the difference by looking at the bottom of the casing on a cartridge. If there’s a circle centered in the bottom of the casing, it’s a centerfire cartridge. If the surface is smooth and one solid piece, you can assume the cartridge is rimfire.
Common Rimfire Cartridges
Popular rimfire cartridges used today in addition to .22 LR are .22 Magnum also known as WMR (Winchester Magnum Rimfire) and 17 HMR (Hornady Magnum Rimfire).
.22 WMR or 22 Mag as it’s commonly known, has more than twice the muzzle energy of .22 LR with 314ft-lbs. The .22 WMR cartridge is extremely popular for varmint hunting since it has more energy at longer range than .22 LR. The .17 HMR was born from this arena by necking down the .22 WMR to .17 caliber and using a 17-grain bullet instead of a 40-grain. In standard loads, this develops 2550 feet per second (fps). That’s 600 fps more than a .22 WMR and 1300 fps more than a .22 LR.
How it’s Produced
Rimfire cases and bullets are made similarly to centerfire ammunition. Daniel Compton, Product Manager of Rimfire and Shotgun ammunition at Federal Premium® was kind enough to offer his time to tell me a bit about the production process. He said,
“Bullets are typically pressed from lead wire into either round nose or hollow point variants. The bullets can be lead, copper, nickel, or even polymer coated. Cases start as rolls of metal sheeting and are then punched out, drawn longer and then finally given a distinct rim on the head.
Rimfire differs from centerfire as the priming compound is placed directly in the bottom of the case rather than in a separate primer cup. Priming is done with the priming compound wet to prevent any chance of detonation during the priming process as it is a very sensitive compound. The rimfire case is then “spun” in a centrifuge to move the priming compound evenly around the rim of the case. This is important as the priming compound needs to be in contact with the case rim so it can detonate at any point on the rim where the firing pin could strike.
After drying, the primed cases are then charged with powder and the bullet is inserted. The final step is to dip the bullets in a coating material to prevent an excess of lead build up in barrels.”
Bullet Design
Traditional cast lead centerfire cartridges have a constant bullet diameter after the taper from the nose that extends to the base of the bullet case. They also have grooves for lubrication closer to the base that are covered by the case wall when fully seated. Most modern rounds utilize copper or other jacket materials without the need for lubrication, but they still have the same constant bullet diameter. In contrast, the bullets of most rimfire cartridges do not have a constant diameter. The external portion of the bullet has a larger diameter and the portion inside is tapered to fit in the cartridge. This bullet design was created to provide the lubrication on the outside. The exterior portion of a .22 LR bullet is coated in wax, copper, or other materials to provide the necessary lubrication. Cartridge Variations Rimfire, especially .22 LR, has a wide variety of loads and bullet types. There are standard round-nose or hollow-points, but there are also many other options since .22 LR is arguably the most popular caliber in the world. Standard velocity cartridges are just barely under the sound barrier, they’re extremely popular because they are highly accurate and suppress well. After that, there’s subsonic rounds which are most ideal for shooting suppressed. Beyond that, there’s even .22 LR shotshells for short range varmint control. For personal defense, there are higher pressure cartridges producing more velocity, including hollow-point bullets for controlled expansion.
Federal Premium® recently launched the Personal Defense® Punch™, for self-defense. Unlike most self-defense bullets, which are hollow points, these are flat nosed to achieve penetration requirements out of short barrel handguns. While hollow points make a larger hole from the expansion, they dissipate energy too quickly to achieve penetration depth.
As the affection for the caliber grows, manufacturers continue to increase variations. It’s safe to say the .22 LR isn’t going anywhere anytime soon.
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