The .17 HMR will be around in 20 years. It has enough guns in the market place to support the cartridge for a long time. The .22 Hornet and .22-250 will be around quite a while owing, again, to the number of guns, both new and old, available to burn the ammo. The 5.56mm NATO/.223 Rem will be around until Jesus calls his flock home. The 5.45x39 Soviet will stick around forever, too, but finding accurate hunting rounds and reloadable brass could be tough. I don't know of any rifle manufacturer building .222 Remington rifles today, but there are a enough of them around to support ammo makers future production.