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JAN-LODEWYK SERFONTEIN

ON TARGET •••

The abbreviation “ BC ” comes from ballistic coefficient . The BC of a bullet is an indication of how streamlined a bullet is . It measures the ability of the bullet to cut through the air . Mathematically BC is the ratio between the sectional density ( weight / diameter ) of a bullet and its form . The higher the density of the bullet , the better it cuts through the air .

Why do so many marksmen talk about the BC of a bullet and why is it important ? One wonders if it really matters how well a bullet can cut through the air . The air is not the target after all .

Should we not rather look at how well a bullet works once it reaches its target ?

The BC of a bullet is important for marksmen , since it is used to determine the trajectory and wind deflection before reaching the target . With this information , changes can be made to ensure that the target is reached in exactly the right spot . The distance to a target can easily be determined with range finders . However , the wind is a variable which seldom stays the same . For this reason marksmen want a bullet with a high BC which is not affected by wind to a large extent .

If you compare two . 243 bullets with the same weight , their sectional density will be the same since they weigh the same and have the same diameter .

Using the Sierra ' s bullets as an example , the BC ' s of the different

Table 2

Distance

6.5x55 BC . 495 . 2850fps . 140gr . 25km / h .

Vertical Horizontal variance variance

6.5x55 BC . 535 . 2850fps . 140gr . 25km / h .

Vertical Horizontal variance variance

bullets are as follows :

• Sierra Pro Hunter 100 gr . BC - 0.373

• Sierra Game King 100 gr . BC – 0.430

Based on the different BC ' s , the Game King bullet will cut through the air the best and will be the least affected by wind .

How big a difference does the BC of a bullet make in respect of its trajectory and direction on a windy day ? Thanks to technology , computers can answer those questions without us having to be on the shooting range . An example of such a table ( computer generated program ), is reflected in table 1 . The variables entered into the computer were calibre , BC , speed , weight and wind speed .

When we look at a calibre which is known for its good BC , the following information is provided by the program in respect of a 6.5x55mm ( Table 2 ).

It is evident from these examples that two bullets of the same calibre , the same speed and weight with a different BC will reach much the same target up to approximately 300m . When comparing different calibres a better BC bullet will have more advantages .

Is BC therefore important ? As far as the choice of a bullet of a specific calibre is concerned , not really . However , when you have to decide between two different calibres , rather choose the bullet which cuts through the air better and is less affected by external factors .

SENWES Scenario • DEC 2016 / JAN 2017 47