Quote:
Originally Posted by boomtube
SD is a simple math ratio of weight and diameter, no more.
A solid copper or steel jacketed bullet of a given diameter and weight has the same SD as a thin jacketed hollow point with a soft lead core. Meaning SD tells us nothing useful.

You may calculate the SD (Sectional Density) of any bullet by dividing the weight of the bullet (in pounds) by the square of its diameter.
Example: a 30 caliber bullet of 180 gr.
Weight in lbs = 180/7000 = 0.0025714 lbs.
Square of its diameter = 0.308X0.308 = 0.9486
SD = 0.0025714/0.9486 = 0.271
As stated above, if the diameter and the weight are the same the SD is the same.
SD is important because it has a significant effect on penetration. The higher the SD the better the bullet will penetrate assuming equal things like bullet design, materials, impact velocity etc.
A 180 gr. 0.308 bullet will have better penetration than a 180 gr. 0.338 bullet. ( assuming many things being equal like design, impact velocity etc.)
SD for the 180 gr. 0.338 would be: 0.225
I hope that helps on getting a better understanding of SD.
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