Long range is a bit relative, I guess.
We run a long range school here in Virginia, and host practice shoots and long range matches every 4 to 6 weeks... we had a nice shoot today in about a foot of snow...
Anyway, if you're talking about 600 yards, the .223 with a heavier bullet, or the 22-250 with at minimum a 60 grain bullet will get there, but that's about the extent of their ability... and neither will arrive at 600 with a whole lot of power...
The .243 win is a good option, and we trained a guy recently and then helped him build a very low cost long range rifle... he's got less than 500 dollars in the whole rig, scope included...
truck bed paint on the factory tupperware stock... some plumber's putty (an old trick I devised) in the stock hollow... some glass bedding of the recoil lug and a skim bed on the receiver... and some glass bedding in the forearm to keep the barrel free-floating... and she really shoots...
The rifle also performs very well to 1040 yards. The 87 VMAX is a much better long range bullet for the .243 win than most of the heavier options. It's got an honest .400 BC...
I've shot a factory Savage 110 in .270 win, with a cheap 4x scope on it... (see the "ready rifle" mentioned on the bangsteel.com page)... and it works.
The sporter barrel heats up quickly with the .270 pushing the 150's at around 2750 fps, so I don't typically take the 3rd shot... but here are two, at the 24" steel plate at just over 1000 yards... the 4x scope doesn't even need a slanted base to dial that far, as it has 170 MOA of erector movement in it... as an aside, the crosshairs will just *barely* quarter the 24" plate at 1000 yards... but shots like these can be made with some very lowly, unassuming rifles you'd never suspect of being able to do this kind of stuff. The handload is the key...
But you don't want a rifle to heat up too fast on you... so I'd venture back toward the .308 win. It has some advantages over the lesser caliber cartridges that are gaining such popularity these days. First, it's not usually (usually, I said, notwithstanding the current rush on reloading components) hard to get brass and bullets for. The .308 barrel will last around 10,000 rounds, about 3 times what the typical .260 will last, and 4 times what a .243 barrel usually goes. And the .308 hits harder, with heavier bullets. We shoot them regularly against the 6.5's and they can hold their own very well--in the hands of someone who knows what he's doing...
I put together a "gun show" special earlier this year... it was an old Savage 110FP, heavy barreled (long action) .308... I worked the trigger... treated the stock as mentioned earlier in this post... installed some Burris Signature Zee rings (Weaver base compatible) with 30 MOA of offset in them... and I caught a Bushnell Elite 10x tactical scope on sale at Midway for 200 dollars delivered... and I ended up with just at 600 dollar in that rifle... and though I don't have a picture handy, I can tell you that it shoots like this:
So you can come up with something that'll work, if you shop around... even the sporter weight barrels can shoot well--for a limited number of shots--out to 1000 yards. And even heavy barreled guns get hot and "walk" the POI after 3 or 4 shots (depending on the cartridge)...
Good luck in your search.
Dan