Pressure vs Velocity

[Nhoro]

Hi All,

Looking at Quickload and Gordons Reloading tool as well as some reloading data from my range sessions, I have seen a pattern developing and I wanted to start a discussion.

First off I know it is dangerous to assume, no hard and fast rules etc. I just see an interesting trend.

So for my 458 Lott, I see that 3 powders in the same family produce very close velocities at the same pressure. For example Vihtavuori N133, 135 and 140 all top out at the same pressure and velocity is within 1 % of eachother. If you add in N530 which is now a double base powder, you get almost the same velocity at the same pressure. So I checked Somchem S321. Pretty much the same pressure and velocity.

What changes is the case Fill % Slower powders take up more space for the same velocity. Knowing that we aim for at least 80-100 % case fill for efficiency and consistency.

So basically I am seeing that velocity and pressure are related if all else remains equal (same projectile/case volume/seating depth etc) Is this a fair observation ?

 

[Christot]

Not really . The energy transferred to a bullet is the integral of the pressure over barreltime. An exaggerated comparison would be a flat line and an exponential function of pressure where the final pressure at the end of barrel is the same. Energy would be the area under the curve if bullet weight is the same. Do you think both are the same?

 

[Nhoro]

You illustrate my point quite well. How does empirical data from 5 supposedly different powders produce the same pressure and velocity combination.

Obviously, this is similar powder burn rates but still- as you point out- the integral of 5 different curves is within 1 %. All that is varying is powder volume and % load capacity.

Is the barrel length and strength of steel simply capping the energy that can be contained and therefore transferred to the bullet. It seems to me that something is the limiting factor in all of these cases.

 

[Bert the Turtle]

Peak pressure is the limiting factor.

Different burn rates allow for different areas under the curve at a given peak pressure. The big bores tend to use slow powders to keep generating more gasses to keep the pressure up until the bullet exits. If
used something like bullseye at the same peak pressure, you’d get lower velocity because It burns quickly and the amount you can safely burn isn’t enough to keep the pressure up for the full volume of the barrel.

Take it to an extreme and you get the V3. There you had multiple charges along the length of the barrel. As the projectile passed, the charges would fire, adding more pressure to the barrel.

 

[Christot]

I made a comparison for the .458 Lott with N140, N540 and N550 and Swift A-Frame 450gr. All optimized to max. pressure. The differences in energy were quite significant! While N140 delivered "only" about 7100J (700m/s), N540 was about 7700J and N550 about 8100J (745 m/s). Load density of 88% with N140 is not something I'm in favor of. N550 was about 100%.

 

[IvW]

The closer to 100% the better...

 

[Bob Nelson 35Whelen]

Peak pressure is the limiting factor.

Different burn rates allow for different areas under the curve at a given peak pressure. The big bores tend to use slow powders to keep generating more gasses to keep the pressure up until the bullet exits. If
used something like bullseye at the same peak pressure, you’d get lower velocity because It burns quickly and the amount you can safely burn isn’t enough to keep the pressure up for the full volume of the barrel.

Take it to an extreme and you get the V3. There you had multiple charges along the length of the barrel. As the projectile passed, the charges would fire, adding more pressure to the barrel.

@Bert the Turtle
As you so eloquently put it velocity does not depend on peak pressure alone. Velocity is dependent on muzzle pressure not peak pressure. Some powders have a parabolic curve that gives an early peak pressure that drops sharply by the time the projectile leaves the muzzle.
Newer powders like CFE223 and a few others reach peak pressures latter and the pressures drop slower giving a higher muzzle pressure hence higher velocity for the same peak pressure.

 

[Bob Nelson 35Whelen]

@Bert the Turtle
As you so eloquently put it velocity does not depend on peak pressure alone. Velocity is dependent on muzzle pressure not peak pressure. Some powders have a parabolic curve that gives an early peak pressure that drops sharply by the time the projectile leaves the muzzle.
Newer powders like CFE223 and a few others reach peak pressures latter and the pressures drop slower giving a higher muzzle pressure hence higher velocity for the same peak pressure.

Hopefully this explanation simplifies this
Bob

 

[Nhoro]

Ok, I understand the area under the curve until the bullet exits. It still seems to me that at same peak pressure, the velocity difference is less than I would have thought. It seems that the volume of powder varies much more. I would also guess that a limited case capacity like 458 win mag/ lottery will show skewed results. I am sure a 416 rigby would benefit more from the correct powder.

 

[Clodo Ferreira]

Hi Nhoro,

The thing is what Bert The Turtle wrote and what Bob Nelson shows.
The asumption in your first post about "the same pressure and same velocities" with different charges of different burning rates powders is not and exact one, if maximum safe pressure charges are used. If they produce the same velocity the pressures, very likely, will not be the same.
As the powders (inside the same type: single base vs. single base; double base vs. double base and so) yields the same amount of energy per weight, as they burn, in form of heat AND expanding gases (plus other not relevant here), is unlikely that different weights of the same type of powder, but different burning rate=different pressure/time curves, will produce the same velocity, "at the same maximum safe peak pressures". Thats because the energy liberated by the slower ones (more powder by weight for a 100% or even more, load density), for the same case volume-caliber-bullet weight combination, will be greater!
More than probably, maximum safe pressure charges with same type of powders very close in burning rate, wich means weight charges not so different, will not show very different velocities. But, marginaly, they will! How significant is another matter.
As English is not my first lenguage, I apologize for the innaccuracies in my writing...

Best!

CF