Re: 500 yard group
It is possible, I have done it... less than an inch at 500. It wasn't luck and the rig shoots bugholes in good conditions. Testing for drop I fired a group at 300, 400 and 500 yards (3 shots) consecutively and groups were (at 300 I forget???) but about 1.1" at 400 and .980" at 500. The 300 yard was very small, smaller than the others I just forget but, it's on the pic I took of the targets some place.
I've shot several 2-3" groups at 800, and a nice 7 shot group at 600 once with the 200gr Accubond random load that measured under 2.5", one was a flyer 1" out to the right too, so really it was a 1.5" and a flier for the whole story and those were fired super fast before dark 30 just to compare the drop to the 210 JLK. This is with the 28 lb. bench rifle in 30/338 Lapua Imp.
500 yards is 5 times as far as 100 yards so group dispersion would be five times that of the 100 yard, or 1.25", in perfect conditions. Due to wind, velocity and other variables I factor in about a 5% increase in group size for every 100 yards beyond 100, 20% at 500 and 45% at 1000 seems reasonable and fits pretty well with my experience.
In a nut shell, I believe a rifle is capable at 500 yards to hold it's 100 yard level of precision, I've just seen it too much, but still figure on the 5% per 100 rule.