For your first Long Range gun start out with something a little easy to play with.
I have a Model 700 Stainless Tactical 5R Milspec Barrel in a .308. It is in excellant condition, I will sell it to you for $750. it listed at $1250 new. Already has scope mounts, ready to go to the range. I will even through in a NC Star 10x40x50mm Scope. The scope is cheap and I would suggest replacing with a good Leopold or Nightforce for long range shooting. If you would like a new gun go with the 6.5 Grendel a bit pricy but a really good long range gun. With the right optics will shoot rings around the .308. The grendel has a much flatter trajectory at a 1000 yds. If you would like I can get you the cost of one ready to go. email@example.com
If you really like the AR30 go with the Armalite in a .308 or 300 Win Mag I sell them NIB for $1500 with a Free box of ammo.