Complex sentence: This is done because the optical quality of the scope is best in the middle of its adjustment range, and only being able to use half of the adjustment range to compensate for bullet drop is seldom a problem at short and medium range shooting.
Split into simpler sentences: This is done because the optical quality of the scope is best in the middle of its adjustment range. Only being able to use half of the adjustment range to compensate for bullet drop is seldom a problem at short and medium range shooting.