Bullet Ballistics: Pressure
When a primer spits fire into the powder charge and burning commences, gases form, increasing pressure inside the case and (because pressure produces heat), accelerating the burn. On a bullet ballistics graph, you’ll see a pressure peak after a short horizontal line showing the delay between primer detonation and powder ignition.
After that peak, which typically happens within a millisecond (1/1,000 second) after the powder starts to burn, the pressure curve arcs back down. This decline is relatively gradual as the bullet moves forward, increasing the bore volume behind it. The faster the powder, the steeper the curve on both sides. The area under this pressure/time curve translates to bullet velocity. Two to three milliseconds after the striker hits the primer, pressure has dropped to zero. The bullet is on its way.
A 180-grain bullet from a .300 Weatherby Magnum exits the muzzle of a 26-inch barrel about 1 1/4 milliseconds after it starts to move. The following bullet ballistics chart shows what happens (data adapted from a pressure/time curve in the excellent text Any Shot You Want, a loading manual by Art Alphin’s A-Square company).
Bullet Ballistics Chart
|Time (seconds)||Pressure (psi)||Velocity (fps)||Distance (inches)|
|.0005||60,000 (near peak)||1,400||2.80|
Bullet Ballistics: Peak Pressure
A few things to note: First, peak pressure comes when the bullet has moved only about 3 inches, even with the slow-burning fuels appropriate for a .300 magnum. Pressure drops off fast, too, losing 90 percent of its vigor in the next 18 inches of barrel. But the bullet continues to accelerate even as pressure behind it diminishes. Between 14 and 21 inches, pressure loss totals 18,000 psi.
But bullet speed increases 300 fps! With very little pressure remaining at the muzzle, the bullet is still accelerating! The value of a long barrel is clear, even if nearly all of it is used to control the tail of the pressure/time curve.
Bullet Ballistics: Pressure/Distance & Pressure/Time
A pressure/distance curve differs from a pressure/time curve in slope, but it has the same general shape: steeper at the start than at the finish. The area under a pressure/distance curve represents the energy available for the bullet. However, the energy generated is not all available downrange. A lot of it is lost in thermal (heat) transmission, expansion of the case into the chamber wall, bullet/rifling friction and bullet rotation.
Plotting a load’s pressure/distance curve helps designers of gas-driven autoloading rifles because these rifles must tap the gas at some point in the bullet’s travel. Too much pressure, and the slamming can damage rifle parts. Too little, and bolt travel is insufficient to clear the fired case.
Pages: 1 2
About the Author: Wayne van Zwoll is a regular contributor to the Gun Digest annual, and author of the Gun Digest Book of Sporting optics. He is a nationally-recognized expert on rifles, optics and western hunting.
Leave a Reply
You must be logged in to post a comment.