I have multiple varmint/target rifles in calibers from 17HMR up to the 338lapua and bigger, and would have to say that for ammo availability {non reloader} you can not go wrong with 223 or 22-250 and I would strongly recommend taking a good look at .204 Ruger. For shooting longer{again varmint} ranges . look at the 6mm/243 and perhaps some of the 6.5 rounds. A lot depends on how much you are going to shoot at a time and how much you want or are willing to spend doing it.