I experience the same thing Cayugad, Good shooting with iron sights at 50 yards, pretty decent at 75, stink at 100. Put a scope on that sucker and I have no problem shooting tight groups way out there.

I have a theory (may be total hogwash). I think that because I can see my 3" dot pretty well at the closer distances I tend to hold the sight picture better through the shot (follow through). But because I have a hard time getting that 100 yard sight picture I let it wander as soon as I pull the trigger.