Ballistics
BP Pressure Curves & Bullet Obturation
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Before we get to the actual Ballistics & Handloading section of the website, lets review the BP vs Smokeless pressure curve for the 45-70.
BP Pressure Curves & Bullet Obturation
By Wayne McLerran
5/20/15
For years I’d read claims about how much better black powder (BP) was at obturating (expanding) cast bullets compared to smokeless powder due to the faster rising edge (rise time) of the BP chamber pressures. Some shooters suggested that BP burned an order of magnitude faster than smokeless which I found hard to believe but was unable to find data to confirm or disprove it. Chamber pressure curves are relatively easy to find for smokeless loads but not for black powder loads. Hence, I could not find actual data to support the claims. That changed when I found the Oehler 43 chronograph pressure curves below comparing three grades of Goex BP and IMR4759 smokeless powder in a .45-70. Note that all the Goex loads utilized a 500gr cast bullet in a 23.75” barrel. The IMR4759 load used a 410gr cast bullet in the same barrel length. Also note that although it takes approximately 2ms (milliseconds or 1/1,000 of a second) after the primer ignites the powder for the bullet to exit the barrel, rise times are measured in µs (microseconds or 1/1,000,000 of a second).
You’ll notice that the Goex rise times are essentially twice as fast as the IMR4759 rise times. I’ve seen numerous pressure curves of various smokeless ammo rifle loads and typical rise times were in the range of 180µs to as much as 400µs. So, there should be no doubt that the burn rate of black powder is significantly faster than smokeless, although not by an order of magnitude but rather by factors ranging from 2 to 5. Consequently, it should stand to reason that BP is more efficient at obturating the relatively soft cast bullets shot in BPC rifles.
By the way, in the electronics industry rise time refers to the time required for the leading edge of an electronic signal to rise, and time is typically measured between two specific points on the leading edge (front) portion of the curve. For firearm pressure cures the typical low value point is at 25% of the maximum signal height and the high value at 75% of the maximum signal height. The Oehler 43 measures the rise time based on 25% & 75% values.
If you’re wondering what electronics has to do with pressure curves, they’re generally measured and displayed using electronic equipment. Strain gauges are used to indirectly measure pressure levels over time by converting the amount a firearm chamber stretches due to pressure into an electrical signal. Specialized electronics converts the strain gauge signal into a trace or curve that can be displayed on wave form recording equipment such as an oscilloscope or analyzed and displayed on a computer with dedicated software. For some examples of pressure curves (traces), although for smokeless powder, go to the following site: http://www.shootingsoftware.com/pressure.htm
Bill Knight, considered an expert on black powder manufacturing, brought up a good point that we should not forget when comparing black powder and smokeless powder pressure curves. Bill said, “Black powder burns at essentially the same rate regardless of increasing pressure in the chamber or cartridge. Therefore, the burn rate and pressure curve rise times are independent of the pressure levels.” Smokeless powders are generally progressive burning powders, meaning that as the pressure increases the burn rate increases. So, the leading edge of the BP pressure curve will rise rapidly in a linear fashion compared to the leading edge of smokeless pressure curves, which will have more of a non-linear shape and rise slower. But smokeless powders are capable of reaching much higher pressure levels than BP.
When looking at firearm pressure curves, many estimate the point in time that the bullet exits the barrel. Assuming the bullet properly seals the bore, the area under the pressure curve from the point of ignition to the time the bullet exits indicates how much energy is imparted to the bullet, resulting in velocity. Therefore, if BP and smokeless loaded cartridges with the same bullet are fired in the same rifle and are loaded to attain the same velocity (key point), the area under the pressure curves from the point of ignition to the time the bullet exits will be the same although the shape of the curve will be different.
Wishing you great shooting,
Wayne
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