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
.223 Calibre although .223 Will fit on a 5.65 mm, a 5.65 mm Will not fit in a .223 calibre chamber.
More than likely your barrel will say .22 Long Rifle only. So, you can use any .22 Long Rifle ammunition.
.22 Short, Long, or Long Rifle.
7.62x54(R) rimmed rifle ammunition.