Help me understand…

[Littlehud]

I know there are some very knowledgeable people on this forum, so I will put this out here.

I am a bit confused about some of the load data I am seeing for different powders. Using the same bullet and cartridge, but different powders, I see different velocities and pressures, but not exactly what I expected. As an example, I have the load data for a .270 Winchester for IMR 460 (101), Hodgdon Hybrid 100V (128), Winchester StaBALL 6.5 (129), and IMR 4831 (132) powders. The numbers in parenthesis are the rank number in the burn rate chart.

The confusing part is this: H 100V and StaBALL 6.5 have similar burn rates, being #'s 128 and 129 on the chart. They give similar velocities with similar pressures, just as one would suspect. However, 4064 and 4831 also give similar velocities with similar pressures but are far apart on the burn rate chart at #'s 101 and 132 respectively. Why so similar when the burn rate is so different?

The other thing that confuses me even more is that looking at maximum loads for the different powders, 100V and 6.5 show 62,300 PSI while 4064 and 4831 both show just under 51,000 PSI. How can there be so much difference for max loads? Why couldn't you use more powder with 4064 to increase the pressure to 62,000 and get higher velocities? There's obviously more to the equation here.

270 WINCHESTER
Case: Winchester
Twist: 1:10.000"
Primer: Winchester LR, Large Ri e
Barrel Length: 24.000"
Trim Length: 2.530"
BULLET WEIGHT
140 GR. SFT SP

Starting Loads Maximum Loads

Powder Bullet Diam. Grs. Vel. (ft/s) PSI Grs. Vel. (ft/s) PSI

IMR 4064 0.277" 42.0 2,608 43,300 46.1 2,828 50,800

IMR 4831 0.277" 50.6 2,738 44,400 54.0 2,896 50,700

Hybrid 100V 0.277" 50.0 2,787 53,600 54.2C 2,964 62,300

StaBALL 6.5 0.277" 50.3 2,809 50,200 54.7 3,033 62,300

 

[MagnumManiac]

Burn rate charts are a guess at best and have virtually zero correlation to how a powder burns in a given CARTRIDGE.
Burn rate is NOT CONSTANT, this is why powders on a burn rate chart do not correlate with the same velocities or pressures. Also, there is no set STANDARD to which each manufacturer sets the base powder, so they can be very different to what's listed.
An example, straight wall cartridges like the 458 Lott exhibit best performance with middle burn rate powders that are fast for the case volume, but this is due to the nature of straight walled cases causing a powder to behave much slower in burn than in a bottle necked case.
If you try to burn a slow powder in a straight walled case, the pressure doesn't develop because the bullet outruns the pressure/gas volume and the powder fizzles and dies out. If there is not enough start pressure, the powder can't catch up and burn fast enough to fill the void behind the bullet, just like an over fuelled cylinder, it just can't burn in the volume provided.

I would place very little validation on burn rate charts.
The easiest method to determine a suitable powder is to look at several load data sources and choose the powders that are listed most frequently in the top 3-4 that produce the highest velocities, this means that they're the most efficient powders for that cartridge.

Cheers.

 

[PowellSixO]

Burn rate charts are a guess at best and have virtually zero correlation to how a powder burns in a given CARTRIDGE.
Burn rate is NOT CONSTANT, this is why powders on a burn rate chart do not correlate with the same velocities or pressures. Also, there is no set STANDARD to which each manufacturer sets the base powder, so they can be very different to what's listed.
An example, straight wall cartridges like the 458 Lott exhibit best performance with middle burn rate powders that are fast for the case volume, but this is due to the nature of straight walled cases causing a powder to behave much slower in burn than in a bottle necked case.
If you try to burn a slow powder in a straight walled case, the pressure doesn't develop because the bullet outruns the pressure/gas volume and the powder fizzles and dies out. If there is not enough start pressure, the powder can't catch up and burn fast enough to fill the void behind the bullet, just like an over fuelled cylinder, it just can't burn in the volume provided.

I would place very little validation on burn rate charts.
The easiest method to determine a suitable powder is to look at several load data sources and choose the powders that are listed most frequently in the top 3-4 that produce the highest velocities, this means that they're the most efficient powders for that cartridge.

Cheers.

Excellent information right here.

 

[sedancowboy]

There is no correlation to a burn chart. Two powders next to each other may be the same burn rate and the very next powder might be much slower if you are going up in numbers. It is dangerous to assume that just because two powders are next to each other on the burn chart they will act the same or work in the same cartridges.
MagnumManiac said it well.
[h4][/h4]

 

[codyadams]

In addition to what MagnumManiac said, another thing to look at is case fill. Generally, the powders that provide near 100% case fill will also provide the best velocities and consistency, but this isn't set in stone. Another aspect to velocity that is unrelated to burn rate is energy potential. For instance, Reloder 26 and H1000 are very close on a burn rate chart, but you will essentially always get better velocity with rl26. The denseness of the powder also comes into play, in my .260 AI, I can easily fit 49 grains of rl26 for my accurate load, and reach max pressure around 51, however when I tried H1000, I never was able to reach max pressure before I was out of case capacity, and my velocity was almost 200 fps lower than rl26, but yet they are very close on a burn chart.

There is way more information on this subject than I know for sure, and just keep an open mind and realize that not all things correlate to one another. Things can get complicated when working with wildcats or certain very low engraving pressure bullets like the absolute hammers, that can throw even a very educated reloader for a loop.

As far as pressure differences, I have seen it where they come out with a new powder and it gives these amazing velocities, but when you look deeper into it, there is 10,000 psi of difference between the new powder and the old one they are comparing to. Part of me thinks it is simply advertising, so they can say this powder does so much better, when it's just a higher pressure load. Others say that it is because certain powders get less stable at the higher pressures. Bottom line, it is vitally important to learn how to read pressure signs. If a load is listed at only 50,000 psi, there is a fair chance you can load higher, but only if you know what your doing and what to look for so that you can stay safe and keep all your digits.

 

[GLTaylor]

All of the above is correct and well stated. Only thing I can add is that some of the high energy powders can begin to act a little erratic when you get up near higher end pressures while working up a load. VV N540 is classic. Shoots good and clean, but when you get near max loads, pressure can really spike and startle you!

 

[sedancowboy]

All of the above is correct and well stated. Only thing I can add is that some of the high energy powders can begin to act a little erratic when you get up near higher end pressures while working up a load. VV N540 is classic. Shoots good and clean, but when you get near max loads, pressure can really spike and startle you!

Ditto RL-26 when you get to the top that last 1/2 grain can scare you.

 

[Gunpowder]

I think a lot of the pressure differences you're seeing is the old standard of measurement vs new. Like you noted, all the newer powders were 60+ psi. Look again at 4064 and 4831, does it say psi, or cup? Before modern pressure testing equipment, they used the copper crusher system, which used little copper discs or cups as I remember reading it. So when it says 50k CUP, that's Copper Units of Pressure.

 

[Canadian Bushman]

There is also retardant coatings and kernel size. Couple that with fill ratio, case shape/capacity ( as stated by others ), and even the particular rifle that was used that day. Burn rate is just one variable. Not one I would lean on very much.

There is a lot going on in there.

 

[Littlehud]

Thanks to all who have responded. The answers seem very reasonable as explanations. If I understand correctly, it would seem 4064 must become erratic if more powder were used to increase pressure, whereas 4831 would overfill the case since it is a compressed load already.

as far as burn rate goes, each powder may react differently depending on case capacity fill percentage and/or case shape. Between these two things (and possibly other factors) it explains the seemingly odd result for load data.

If this is not correct or there is more info please feel free to share. I enjoy learning all I can.

 

[MagnumManiac]

You are correct in your summation.
The reason some powders are listed with lower max pressures is due to them being erratic at max pressure and a slight increase in powder causes a way over max pressure event.
Also, double base powders produce more energy for a given weight and become unstable as heat and pressure get beyond their max threshold. This can cause a normal pressure load to go sky high in the heat of summer or if the rounds have been exposed to a heat source...I found this out here in Australia and it ruined my paid hunt with a rifle with a bolt locked solid.
RE22 was the culprit. Swollen case head, no primer was ever found and the bolt face was torched. Had to bash the bolt open and then bash it rearward to extract the case, luckily it was a CRF action and not a Rem 700!

Cheers.

 

[Trnelson]

Another factor that influences final performance is volumetric energy. Other wise stated as energy density or Joules per Gram (J/G). Just because two powders are close in burn rate the final performance can be substantially different due to one powder having greater amount of overall energy released.