The bullet is a kinetic energy transfer medium. It has energy due to its movement. It gives us that energy on striking and penetrating the target. Primary transfer of energy is the goal in defensive, hunting or material destruction, but is of less importance in targeting paper. As the bullet passes through the atmosphere it is subject to negative acceleration (for some ‘deceleration’) due to atmospheric drag.
Components of Bullet Drag
Atmospheric drag has three major components.
• Bow Wave
• Base Drag
• Parasitic Drag
Mach Speed Number Calculator
The speed of sound is Mach 1, or about 343 meters per second, at 200C. That translates to about 1126 ft/sec or 768 mph. The speed of sound is divided into subsonic, transonic and supersonic, the velocity regions of importance to the shooter. Subsonic is less than 273 m/s (meters per second) or Mach 0.8, transonic is from 273 m/s to 409 m/s or Mach 0.8 to Mach 1.3. Weird things happen to bullets in this area, that affect stability as the center of pressure starts to recede from the tip. Supersonic is from Mach 1.3 to Mach 5, up to 1,700 m/s. Most, but not all bullet velocities in rifles run from about 1300 fps to 4000 fps, in SI units that would be a high of 1219.2 m/s. The Mach number in any media is the projectile speed compared to the speed of sound in that media.
Bullet Bow Shockwave
The bow wave is the primary force acting to slow the bullet. To create the bow wave energy is transferred to the air. This means the bullet is losing energy and slowing. At subsonic velocities the pressure wave travels ahead of the bullet. At supersonic velocities the ambient air cannot move around and ahead of the bullet fast enough creating a compression wave as a bullet passes your position as a cracking sound, mini supersonic boom. The things to consider in point design, the sharper the point the lower the bow wave drag. More drag, the less efficient the bullet. Drag at subsonic velocities increases at the square of the velocity, double the velocity the drag goes up four times. Yaw increase bow wave drag, which increases faster than the square of the velocity when in the supersonic range. Waves are created at every boundary change, the base in a flat base bullet and the transition in a boat tail design. Another wave is created at the end of the turbulence following the bullet.
Boat Tail Bullet VS Flat Base
Base drag, as the air passes over the surface of the projectile it drops into the area at the base, creating a turbulence that creates a partial vacuum at the base. The vacuum pulls at the base retarding forward velocity. Boat tail bullets bullet design helps the transition of the laminar flow of area around the bullet and reduces the area affected by base drag. The ogive, or curve the bullet is affected as well by this base vacuum increasing the drag of the ogive.
Bullet Skin Drag
Parasitic or Skin Drag. As the air flows across the bullet there is a boundary layer created between the surface of the bullet and the laminar flow is disrupted creating a surface drag on the projectile. Coating do nothing to alleviate parasitic drag, but grooves, rings and cannelures aggravate the problem. Parasitic drag contributes the least to bullet retardation.
Custom Shooting Ranges for Firearm Practice & Training
Add them up and you have the ‘Total Drag’ on the bullet. The bow wave interjects about 60% of the drag with base drag at about 30% and parasitic drag contributing about 10% to the total drag.
Drag is the bane of ballistics. On the moon, with no atmosphere and dealing with only gravity acting on the bullet, bullets could be square and go as fair given the same bullet weight and velocity. Shooting Range Industries designs and fabricates custom shooting ranges so you can practice shooting with two feet firmly on the ground! Contact us to learn more today.