Ballistics can be somewhat frustrating to grasp. One of the fundamental concepts is the ballistic coefficient or BC. The BC is calculated from the section density (SD) and the form factor (i).
BC = SD/i
Ballistic Coefficient Form Factor
What and how is the form factor determined. 1936 DuPont printed 10 charts that depicted various points and ogives shapes that progressed logarithmically. These profiles where used to compare a physical bullet to the point profile in the chart. This was the form factor. Gâvre Commission back in France of 1881 came up with a standard projectile and the ballistic coefficient was compared to this standard which was a flat base design. The BC was the ratio of the standard bullet’s performance to the comparison. Most BCs are less than 1, though some do exceed this. Our modern problem for the long-range shooter is that we tend to use low drag bullets with boat tails, and these don’t match well with bullet drop charts.
Doppler Radar Form Factor
Today the form factor is tested using doppler radar and other range instrumentation. A new standard is the G7 model closely matches true trajectories of these modern bullet profiles. Because of this the (i) or form factor or the drag coefficient is used to model bullets like other aerodynamic shapes like jets and missiles. The drag coefficient is determined using an ideal standard compared to the actual bullet. But unlike BC, which is better the higher the number, the drag coefficient is better for the lower value. The lower the number the more efficient the shape of the bullet with less drag during flight. The drag coefficient is a dimensionless number and is the amount of drag that a bullet experiences at a given velocity. The more streamlined shapes have a lower drag coefficient at supersonic velocities and that changes rapidly with changes in velocity. Lower the drag coefficient the less drop in its velocity over time. Drag changes with velocity
i = cd /cd(std)
Where I is the form factor. The cd is the drag of a bullet at a velocity. The cd(std) is the drag of the standard bullet (G1 or G7) at the same velocity.
The interesting thing is that as the projectile slows the drag increases causing a drop in the BC until around 2000 feet per second, and as it approaches transonic velocity at about MACH 1.2 down to MACH 0.8 The drag drops suddenly and smooths out at sub-sonic velocities. Air gun and pistol projectiles spend most of their journeys in the subsonic region. Short stubby projectiles with short ogives, they perform well a lower velocity. But low-drag bullets with long ogives and boat tails have little or no advantage at subsonic velocities.
Ballistic Coefficient in Bullet Selection
But the BC is not a tell all for bullet selection. High BC’s are important to the long-range shooting enthusiast. The more ‘slippery’ in air a bullet is the better. For the average hunter engaging game at less than 300 yards it is not as important. Round nose bullets have terrible BCs and high form factor, but these are designed for hunting in heavy brush at 150 yards or less and perform admirably in bucking brush and bringing the game. So, bullets must perform to function, and there is a place for flat base and short orgives, rather than the BC or form factor.
But for the bench rest shooter high BCs and low form factors are the norm.