This could be a really dumb question, but I'm a little confused.  I have a Bushmaster 16" carbine with a LaRue BUIS.  I've zeroed the carbine at a 50 yard range using the Maryland AR15 Shooters Site "Improved Battlesight Zero" instructions (http://groups.msn.com/TheMarylandAR15ShootersSite/improvedbattlesightzero.msnw).

I got the carbine zeroed at 50 yards (I had to crank the front sight way up as I was shooting too high. I almost went beyond the detent's holding ability, and had to crank back down some).

When I shoot closer at 25 yards, however, I'm consistently shooting much lower.  It makes me want to crank the front sight lower to adjust the aim -- but then the 50 yard zero is going to be off.

Am I missing something.  Don't be afraid to smack me upside the head if need be...    

Since the sights are way above the line of the bore, the bullets will only strike halfway between those two extremes if you're zeroed at 50 yards. I'd bet you're hitting about
1-1/8" low at 25 yards? You can't be dead-on at all ranges anytime the sights are higher than the bore line.