.243 is fine except that it burns barrels if you shoot a lot. Use premium bullets. 6mm BR is another 6mm option that is fairly popular (with handloaders). BTW http://www.6mmbr.com/ is a fun site for cartridge info. .308 is a do-it-all cartridge, can hardly go wrong with it except possibly for boredom. .30-06 and .270 Win are both good too, if a bit old-timey (not a bad thing). 6.5 Creedmoor is up-and coming, shoots flatter than .300 WM but won't kill you with blast and recoil. .260 is roughly equivalent. I have a soft spot for .358 Win but it is on the way out if not already there. Stay away from magnums unless you are a masochist. Most of the bolt guns you see for sale on armslist are magnums. There is a reason for that. There are too many tiny variations of cartridges; it is getting ridiculous these days. I have settled on 3 main bullet diameters: .308, .264 and .224. I am trying to purge the others from my collection. This simplifies life when buying bullets. If you are starting off, you might want to give a thought to where you want to end up in 20 years. I got into too many obscure calibers when I was younger. If I were looking I'd get a 6.5 CM.