It's a warning to the user that the only compatible ammunition for that rifle is the .22 Winchester Magnum Rimfire (22 WMRF), and other types of .22 ammunition are NOT compatible.
Why is there a shortage of .22 ammunition?
22 250 Remington
.22
No one that I am aware of.
No, firearms chambered in .22 short, .22 long, .22 magnum, .17HMR, etc... are all currently being manufactured. These all use rimfire ammunition.
22 Short, 22 Long, or 22 Long Rifle rimfire interchangeably.
air rifle
More than likely your barrel will say .22 Long Rifle only. So, you can use any .22 Long Rifle ammunition.
.223 Calibre although .223 Will fit on a 5.65 mm, a 5.65 mm Will not fit in a .223 calibre chamber.
The High Standard R-100 revolver uses .22 Long Rifle (LR) cartridges. This versatile caliber is widely available and suitable for various shooting applications, including target shooting and small game hunting. The revolver is designed to accommodate both standard and high-velocity .22 LR ammunition.
.22 Short, Long, or Long Rifle.