Complex sentence: Ranging is either done using the traditional method of "visualising" the number of yards separating you from the target or, more scientifically by using a telescopic sight fitted with a "mil-dot" reticule.
Split into simpler sentences: Ranging is either done using the traditional method of "visualising" the number of yards separating you from the target. Another, more scientific method, is to use a Telescopic Sight fitted with a "mil-dot" reticule.