Sectional Density is a simple calculation being the ratio of the bullet's weight in lbs to its diameter in inches squared.
E.g. for a 127gr .264 cal bullet: 127gr * 1/(7000gr/lb )*1/(.264^2) = 0.260
It has units of PSI so it must mean something right? Kind of if you're trying to calculate a bullet's BC, but even then it's a dumb-downed variable to make the calculations manageable. A bullet in flight has an "angle of attack" that's not parallel to the direction of flight. For the hunter concerned with terminal ballistics, particularly bullet penetration, SD meaningless!! From the definition of SD, the value is only constant in the direction of the bullet's long axis and only for the bullet's original and unaltered geometry i.e. constant mass and constant diameter. Some on this forum (one in particular) throw SD around in the hunting context as if it has meaning. For the target shooter it's meaningless since BC absorbs a "close enough" value for SD. For the hunter it's at best simplistic and at worst erroneous. When a bullet contacts an animal does its diameter remain constant? Does its mass remain constant? Does it only move in a direction indicated by its long axis? In short, no. Bullets expand, bullets tumble, bullets lose mass, bullets deform. Comparing bullets for hunting based on SD is ridiculous and ignorant.
This is just my opinion based on hunting, retrieving bullets from gel tests/berms, and the destruction of a few brain cells.
E.g. for a 127gr .264 cal bullet: 127gr * 1/(7000gr/lb )*1/(.264^2) = 0.260
It has units of PSI so it must mean something right? Kind of if you're trying to calculate a bullet's BC, but even then it's a dumb-downed variable to make the calculations manageable. A bullet in flight has an "angle of attack" that's not parallel to the direction of flight. For the hunter concerned with terminal ballistics, particularly bullet penetration, SD meaningless!! From the definition of SD, the value is only constant in the direction of the bullet's long axis and only for the bullet's original and unaltered geometry i.e. constant mass and constant diameter. Some on this forum (one in particular) throw SD around in the hunting context as if it has meaning. For the target shooter it's meaningless since BC absorbs a "close enough" value for SD. For the hunter it's at best simplistic and at worst erroneous. When a bullet contacts an animal does its diameter remain constant? Does its mass remain constant? Does it only move in a direction indicated by its long axis? In short, no. Bullets expand, bullets tumble, bullets lose mass, bullets deform. Comparing bullets for hunting based on SD is ridiculous and ignorant.
This is just my opinion based on hunting, retrieving bullets from gel tests/berms, and the destruction of a few brain cells.
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