Re: Out-of-the-box <800yds .308?
My good friend has an SPS Tactical.
He had a bolt knob installed. I Gun Kote'ed the barreled action for him. I sold him an old H-S Precision stock I had laying around that was skim bedded for a different rifle. I painted it in GAP camo. I loaded him 75-80 rounds of 155 A-Max, RL15, Rem 9 1/2s in once-fired Fed GMM brass. It shot 1/2 MOA and under.
We did some shooting over the weekend out to about 650 yards on 14" disc blades that had been worn down to about 12-13".
I used out JBM dope and spotted for him. It didn't shoot quite as flat as the load manual suggested it should have. I don't know if it was the BC is lower than Hornady published or his barrel is a bit slow. Either way, hitting the disc blades was NO PROBLEM with every single shot once we got his dope figured out.
So to the OP:
If you get a good rifle with a descent stock, the rifle will be "effective" for ringing steel at 800 yards no problem, providing your skill level is there. If you have somebody to show you the ropes, it will be much easier. My buddy is new to long-range shooting and had he not had me spotting and calling his shots it may have been a bit more difficult. But together, we had him ringing the steel with boring regularity.
You may not be winning F-Class matches with an out of the box rig, or be benchrest competitive, but you'll certainly kill game and have fun at the range.
Here's a photo from Sunday's shoot:
I would highly recommend getting an SPS Tac or Varmint, get the stock you're most comfortable with, a good trigger, get it bedded, possibly recrowned and you'll have a great rig that will get you shooting.