to the tallest of the rimfire .22 cartridges
the .22 long rifle
is the most popular
cartridge in the entire planet.
Some find that hard to believe especially when all they see and hear in the movies are .357 Magnum, .45, .44 magnum and guns like the Desert Eagle .50 and the Glock. But because of its very affordable price, decent velocity and range, manageable recoil and pinpoint accuracy the .22 lr is usually the round used in every person's rite of passage into the shooting world.*
The .22 lr is also the round used in olympic target shooting competitions.
The .22 lr has many smaller cousins like the .22 colibri, .22 short, and the .22 long (smallest to largest).
The nice thing about the above mentioned .22 rounds are that they all have the same diameter case and same diameter bullet meaning a gun designed for the .22 lr can also fire the smaller rounds. A gun specifically designed for the smaller rounds should not be used to fire the bigger rounds because it is either that the round will simply not fit in the chamber if it is a revolver or more likely might generate too much pressure and may damage the firearm. Although these days, nobody makes guns that are only for the smaller .22 rimfire cartridges anymore.
So with the same .22 lr gun you can use .22 colibri for plinking cans at short ranges, .22 short for blasting away rats, the .22 long for even bigger rats, and the .22 lr for long range target shooting or killing even bigger varmints/pests.
For even more power from a rimfire cartridge in .22 there is the .22 WMR aka .22 magnum.**
Another interesting bit of trivia is that the NATO standardized infantry rifle cartridge (the 5.56mm x 45 is also in fact a .22 caliber bullet. Its bullet is heavier by about 15 - 20 grains than the .22 lr's and travels in excess of 2200 fps over typical .22 lr velocities though! Makes for nasty wounds with its terminal ballistics.
.22 lr only (idea) Moved from there to here
(idea) by karfung
Not to be taken as self diminution, these words are engraved on rifles or pistols that are chambered for the .22 lr cartridge to signify that it is only for use with .22 lr ammunition.
No no, wait.
You can read what I wrote about .22 lr or just listen to my explanation below.
.22 lr guns can, in general, also fire .22 long, .22 short, and .22 Colibri cartridges. It is only when you have a semiautomatic or fully automatic (yes there are full auto .22 lr guns, they are a cheap and fun way to experience fully automatic fire) action does it not become wise to try these smaller and weaker cartridges as they will most likely not cycle properly leading to jams which will then lead to missed shot opportunities and much fiddling with charging handles to get back in action.
I wrote this because when I showed a friend my rifle and he read this, he goes: "Pfffft, why'd you spend money on junk like this, look it even says .22 lr only."
* - Not me though, I remember my first experience of taking a whiff of burning smokeless powder and feeling the kick of recoil was from a chinese copy of the TT 51 in 9mm x 19 back when I was nine or ten. I never got to handle a .22 lr until I was about 20.
** - the smaller .22 should not be fired on .22 magnum guns, they are slightly different in diameter.