Re: Starter caliber for distance shooting...
I would get a .308 built on a long action. This can then be rebarreled up to .30-'06 or down to .260, while retainimg the same boltface diameter (nominally .473"). The .30-'06 can be loaded to reproduce some of the 'lesser but equal' .300WM loads, and the .260's trajectory will generally reproduce the same curves, etc., as the .300WM, while delivering <span style="font-style: italic">more</span> energy than the .308 at the longer distances. If you want o get the most out fof the .30-'06 case capacity, I'm finding quite a lot of that with the .280 rem, essentially a 7mm-'06, delivering supersonic 1000yd performance out of a 150gr bullet
Brass for the .30-'06 is simply ubiquitous; if you can't find any, you're looking in the wrong place. 260 brass can be bought OTC or made up by simply running .243 or 7-08 brass through a regular .260 Rem F/L die. I do that myself with Win brass, no hardships needing to be endured.
I know there can be a fascination about shooting smart recoilling rounds like the .300WM, but unless you're needing to anchor ripsnortin' beasts with astounding alacrity, the 'lesser' chamberings are just as accurate, a tad less expensive to shoot, and simply bypass the recoil issue. If you really, really gotta anchor something majestic, the .35 Whelen will fit into the same mechanical footprint as the .30-'06, essentially. That oldy is still a flat-out goody.
I have been forced to observe friends while they indulge their preferences for the 'big ones', only to find themselves being outscored by something a good bit more tame. If I really thought there was some uniquely unbeatable advantage to enduring that additional recoil, I'd be riding up front on that bandwagon. But there isn't, as far as I can see.
Meanwhile, until then, the .308 is ideal as a starter chambering.
If you don't have a reliable basic bolt action .22LR, I'd strongly suggest one. That's yet another ideal tool for the working out the basic nuts and bolts of marksmanship development. That task is never complete.
Greg