T Inman,
Have mentioned this before, several times, on the Campfire, but if a bullet groups well at 100 yards it will group well at longer ranges, because contrary to popular belief they do not "destabilize" at normal hunting ranges (even, say, beyond 400 yards) just because velocity drops.
But there are other reasons they can shoot less accurately. First (and probably foremost) is wind-drift, which increases at about twice the rate of range.In other words, if a bullet drifts two inches at 200 yards in a certain wind condition, it will drift about 8 inches at 400 in the same conditions.
Another reason is parallax in the scope, combined with inconsistent head position behind the scope.
Yet another is inconsistent shooting.
But there is NO ballistic reason for a certain bullet to group "bigger" at longer ranges other than wind or parallax.
There was recently a long thread on "Ask the Gunwriters" about this, where a guy was shooting sub-inch groups at 100 and 3-5" groups at 200. He could not figure it out, until somebody suggested one of the sling swivel studs might be hitting the sandbags during recoil. Which turned out the be exactly the case, after hundreds of posts and lots of suggestions.
