If the barrel is okay and the trigger group clamps the stock properly with some authoritative squeeze and the receiver heel is properly firm on the stock and the front of the stock pulls down on the barrel band with as-designed tension and the gas piston is tight or shimmed (the figure-8 nut should hit between 4:00 and 5:00 and tension in to 6:00) and nothing is binding the op rod and the flash suppressor is not rattling (vibrations) and the suppressor has been reamed to about .40 at the front hole AND your scope mounting is up to snuff,
assuming a decent optic
and using old M118 ammo, you can count on 2 MOA or better, probably out to 800.
If using M852 ammo or other quality 168-gr loads, 1.5 MOA or better at 800. That 1 MOA group at 100 never has a straight-sided cone of dispersion. If you are lucky or have a match barrel, the .5 to .75 MOA at 100 might be 1 MOA at 800. With 168s at below about 2,500 feet altitude, 800 is about the limit with those bullets.
If using new M118LR or 175-gr bullets, same accuracy as 168s most of the time, but you can stretch it to 1,000 even at sea level and in the cold. Probably 1,100.
M80 ball ammo--2-3 MOA at 100, noticeably worse at 800. M80 ball, unlike M2 ball in the Garand, WILL shoot to at least 600 without going unstable, but I don't know whether it will make 800.
And for you battle rifle snobs, please know that after the barrel, gas system fitting, bedding, and flash suppressor are done, the rest of the National Match tuning for the M14 is directed to assuring *reliability*. And that first list of mods does not "tighten tolerances" or anything in any way that reduces reliability, with the possible exception of a badly done chamber.
Using metal powders in your bedding also makes it possible to take the action out of the stock 5 times a year for three years without wearing anything out or compromising accuracy, too. Don't interpret target shooter caution as evidence of a problem. I wear seat belts while driving, but that's not evidence that I plan on ramming your car if I see you.