TexasMac's Web Site
| BLACK POWDER PRESSURE CURVES & BULLET OBTURATION By Wayne McLerran |
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
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
Posted: 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).
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).