RE: Sighting "in" at 25yds, Where is it at 100yds?
A gun that is sighted in at 25 yards is fine for hunting...AT 25 YARDS!
25 yards, or thereabouts, is a great place to start sighting in a load. I do it all the time. I consult the ballistic charts for the appropriate bullet BC and velocity, and measure the height of the sight from the center of the bore to the line of sight, and see for a 100 yard zero where the bullet crosses the LOS the first time. I then set up my target initially at that range. Usually that happens between 20-30 yards, depending on sight height and velocity.
The point of this is that initially setting up at about 25 yards virtually guarantees that the rounds will hit paper, allowing for initial adjustments. Once I've got it zeroed at 25 yards or thereabouts, I can be reasonably certain that I'm going to hit paper at 100 yards, thus saving me time and ammo trying to get on target. By zeroing at 25 first I can USUALLY get within 4-6" of zero at 100 yards every time, but there are NO guarantees. Like the others have said, the only way to be certain of your zero at 100 yards is to actually shoot at 100 yards.
One of the reasons the 25 yards zero might not be an accurate reflector of the POI is 100 yards is because of bullet stability issues. Bullets often tend not to be fully stabilized within 25-75 yards of leaving the muzzle, yawing and occillating enough to create a spiral-like flight path. It takes a small amount of time to dampen the bullets occilation and allow the bullet to settle into a straight stable flight pattern.
I think it ultimately comes down to an ethical issue. If you sight in a 25 and trust to the ballistic charts, you'll never be certain that you're POI is close enough to your POA to ensure a quick ethical kill. To me, and most of the people who frequent this board, hunting with a rifle whos zero is uncertain is a recipe for a wounded animal, and is therefore ethically unacceptable. You owe it to yourself, the animal and the rest of us in the hunting community, and for the sake of our future as hunters and sportsmen, to ensure your chosen weapon is up to the task of ethically taking game. Failing to sight your rifle in at the range you intend to use it, and wounding an animal as a result, makes you a slob hunter and gives all of us a bad name. I'd ask you to not do this.
Mike