From the Sierra website.

http://www.exteriorballistics.com/eb...ned/4th/44.cfm
2) G 1 is the Most Appropriate Drag Function Choice for Sierra’s Bullets

The tests conducted in 1971 also compared four standard drag functions to determine which of these models was best for Sierra’s boat tail bullets. The four drag functions were published by Winchester-Western:

G 1 - for all bullets except those in the categories below

G 5 - for low base drag bullets (i.e., boat tail bullets)

G 6 - for flat base, sharp pointed, full patch bullets

G L - for hollow point, lead nose bullets

Theory says that if the drag model matches the true bullet drag, the ballistic coefficient will be constant at all velocities. If the drag model does not match the true drag, then we will find different values of ballistic coefficient in measurements at different muzzle velocities.

Several bullets were used in the tests, both flat base and boat tail types. For each bullet type, several test rounds (typically 5 to 10) were fired at each of two muzzle velocities (typically 3000 and 2500 fps). For each test round four ballistic coefficients were determined ( C 1 , C 5 , C 6 , and C L ). This procedure gave two sets of four ballistic coefficient values for each bullet type, one set for the higher muzzle velocity and one for the lower. By comparing the values in the two sets, it was easy to see which of the four C’s changed significantly (outside the measurement error band) and which did not. From theory we expected that the best drag model ( G 1 , G 5 , G 6 , or G L ) for each bullet type to be the one for which the corresponding ballistic coefficient ( C 1 , C 5 , C 6 , or C L ) changed the least amount between the two muzzle velocity levels.

Although we expected G 5 to be best for the boat tail bullets, the tests showed that G 1 was better. This was a surprise. The second surprise was that, while G 1 was best for flat base bullets, C 1 was not constant; it varied significantly with muzzle velocity.

With these observations, G 1 was adopted as the standard drag model for all Sierra bullets. Values of ballistic coefficients in all Sierra Manuals are referenced to the G 1 drag function.

Finding that G 1 was best for boat tail bullets was fortunate, because G 1 has been and continues to be used by all commercial manufacturers of bullets. Consequently, ballistic comparison can be made between bullets made by different manufacturers, as well as between bullets made by a single manufacturer. If different drag models were necessary for different bullets or different bullet styles, comparisons based on ballistic coefficients would not be possible. Another advantage of G 1 is that it is the drag function for the Ingalls Tables, so that ballistic trajectories can be calculated using those tables.