Longer answer: There are more factors at work at 500 vs 100 yds, or more precisely, factors that start having more influence as the range gets longer. Wind, for instance, becomes more of a problem, since it has more time to act on the bullet, and it is also much harder to read the wind(s) over 500 yds vs over 100 yds. Velocity deviations, which may not make much difference @100 yds, may make a big difference at longer ranges, as evidenced by vertical stringing on the target since one round is moving faster, and therefore dropping less, than the next.
These are just some of the things involved that make it somewhat unlikely that *most* people have a low probability of getting a 0.5"/100yd group translate into a 2.5"/500yd group. It's possible, but it takes a lot of work. FWIW, I have a gun or two that can shoot sub 1/2 moa @100yds all day, but I can't even come close to that @500yds. .224 cal guns, yes, but I think some of the problem is btwn the Peltors images/icons/wink.gif
LRH, You are correct, it is very possible to hold .5moa to 500yds and beyond, some actually group tighter at longer range after they stabilize, contrary to what some believe. I have one here at 700yds I shot in the wind at 10-11mph at 10 o-clock with the Ruger 416wby and a 400gn XLC. The wind was very steady and stopped gusting for the 700yd group. The aimpoint was the target to the left of this one and the bull was dead level with the seam on the top of this target. I was shooting to establish elevation clicks nothing else. Variables do add to the difficulty but if velocity swings cause problems they will be relitive to the 100yd group, unlike the wind.