Velocity and it relationship to a bullet’s B/C
- Thread starter Iggyort
- Start date
The Ballistic Coefficient (BC) is dependent on a) sectional density and b) air drag of a bullet.
a) is controlled by mass in relation to cross section and b) is controlled by its shape. For a given calibre (diameter) and weight, BC is only dependent on air drag due to bullet shape (and its stability in flight, which will not be considered here).
The BC is a concept from about 130 years ago, when computing a trajectory was a very labor intensive thing.
So Italian ballistician Siacci invented a method to compute only one single extensive table for an assumed "standard bulllet". This standard bullet (in the Imperial system, Ingalls chose 1 inch diameter and 1 pound weight) was defined as having BC 1.0.
All other (flat fire) trajectories can be interpolated from this table by using a ballistic coefficient describing the relation of the bullet in question to the standard bullet. The BC is simply a scaling factor. A BC greater 1.0 says that your bullet is better than the standard. A BC less than 1.0 says it is inferior to (loses velocity faster than) the standard bullet. The latter is typical for small arms bullets, because the sectional density is much less that that of the standard bullet.
G1 is a well known standard bullet shape, based on experiments, laid down around 1885 by French naval artillery at Gavre in Normandy. G7 is another standard bullet shape, actually originating in the UK around WW2, which is much more similar to modern small arms bullets than French naval projectiles of the 19th century.
In case the actual drag curve of a bullet does NOT match the standard drag curve (a different curve shape cannot be compensated by BC) one can try to use different BCs. Sierra does this by giving 3 or 4 BCs in different velocity ranges.
a) is controlled by mass in relation to cross section and b) is controlled by its shape. For a given calibre (diameter) and weight, BC is only dependent on air drag due to bullet shape (and its stability in flight, which will not be considered here).
The BC is a concept from about 130 years ago, when computing a trajectory was a very labor intensive thing.
So Italian ballistician Siacci invented a method to compute only one single extensive table for an assumed "standard bulllet". This standard bullet (in the Imperial system, Ingalls chose 1 inch diameter and 1 pound weight) was defined as having BC 1.0.
All other (flat fire) trajectories can be interpolated from this table by using a ballistic coefficient describing the relation of the bullet in question to the standard bullet. The BC is simply a scaling factor. A BC greater 1.0 says that your bullet is better than the standard. A BC less than 1.0 says it is inferior to (loses velocity faster than) the standard bullet. The latter is typical for small arms bullets, because the sectional density is much less that that of the standard bullet.
G1 is a well known standard bullet shape, based on experiments, laid down around 1885 by French naval artillery at Gavre in Normandy. G7 is another standard bullet shape, actually originating in the UK around WW2, which is much more similar to modern small arms bullets than French naval projectiles of the 19th century.
In case the actual drag curve of a bullet does NOT match the standard drag curve (a different curve shape cannot be compensated by BC) one can try to use different BCs. Sierra does this by giving 3 or 4 BCs in different velocity ranges.
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