Quantification of powder burning rates.

[El Bibliotecario]

Velocity is measured in feet per second, powder charges and bullet weight in grains, case length in inches...but powder burning rates seems to be strictly comparative; e.g.; "This one's faster than that one but slower than this other one." This may be wishful thinking, but it seems the industry should have a more precise means of measurement. What have I missed?

[MT Gianni]

Burn characteristics may change with bullet weights, primers, chambers and bore size. I also believe the industry fears that someone would go completely linear reasoning that if powder 1x @ 4 gr gives this velocity then powder 1xA would do the same at 3.9 gr. We have the books or read the pressure signs for a reason.
Also one brand is not going to test another brands powder. While there may be standardized testing I have seen too many powders in different positions in burn charts to believe it.

[rancher1913]

also, just because 2 powders are side by side in the burn chart, does not mean they are close in burn, one could be twice as quick as the other but no other powder fell in between them.

[El Bibliotecario]

...because 2 powders are side by side in the burn chart, does not mean they are close in burn, one could be twice as quick as the other but no other powder fell in between...

*laff* Which is one reason why I vainly hoped for a more definitive table than a fast-to-slow ranking. But the reasons given for not having such make sense. I am reassured this is something I didn't miss through ignorance.

[Winger Ed.]

But the reasons given for not having such make sense.

It is confusing.
But the short answer is, 'It's not that simple'.

When they make another batch of a specific powder, it's like Grandma's gravy- it comes out a little different each time.
However; it has to be within about 3-5% of the standard for it.
Otherwise it won't go into the named and labeled cans we've come to know and love.
But that's no big deal.

What is confusing is when you see the listings in a burn rate chart publication from years past--
you'll see about the same list of old and trusted powders---- but some of them may be arranged differently.

Some of it may be a different method used to test them, or maybe some kind voodoo magic going on.
Something as simple as a different seating depth in a given cartridge can change the burn rate and pressure of a powder.

But if you keep with any of the loading manuals and not stray off into uncharted waters--
you won't shoot your eye out kid.

[TurnipEaterDown]

By my understanding, burn speed charts are developed from data acquired in a closed vessel. The vessel is called a "bomb".
Proper parameters of the combustion event are recorded such that a burning speed can be generated.
While I do not KNOW, as I do not do this work, it would make sense to me if this 'burning rate' was a comparison of time to peak pressure comparison, while using a fixed weight of propellant, and some reference powder as the comparison 'target'. We do know that these charts are comparative in nature, and IMR assigns for themselves one of their powders as '100' as a burn speed. I forget which one but it is an older rifle powder.
Likely the material in the vessel is ignited by electric current of fixed amount on a standardized gauge and material of wire to not have a contributing factor in the measurement by any ignition compound.

This is much like calorific quantification of a substance: put it in a closed "bomb", ignite it, measure the temperature rise of the air in the vessel to compute the calorie value of the substance. Food calorie values are actually thermal Kilo Calorie numbers. I suppose they drop the Kilo to avoid scaring people off food or something... (No, seriously, there is some other reason for removing the kilo, but I thought humor would be good.) I believe I remember that the standard assumption of no heat loss to surrounding container is used (adiabatic model) as the temperature rise is rapid and the heat loss insignificant especially if a material such as a good stainless steel is used for the bomb. I remember doing this in laboratory some 30-ish years ago, but didn't do this stuff afterward and so flush the complete memory.

In any event, the firing of a projectile differs greatly, as the bullet moves, and thus the available volume for expansion of the propellant gases changes with multiple factors, such as ignition thermal value & duration, bullet start pressure, bullet mass, friction at bullet - bore interface, etc. As the pressure of the expanding gases changes, the burning rate changes, as the temperature of the gases are influenced by pressure and the rate at which uncombusted propellant ignites is affected by temperature.

Hope that helps in undersatnding the warnings about burning rate charts. They are Not powder substitution charts.

[justindad]

Powder burn rate is a function of pressure, so it’s variable. The effect of pressure on burn rate is different for different pressures. They could publish the burn rate vs pressure, but even that is an oversimplification. Temperature effects burn rate, so now we’re talking about a 3-D graph. What happens when the powder gets compressed in the case? It’s complicated.
*
On the other hand, I could be completely wrong. Could just be the man keepin’ us down!

[charlie b]

No, not wrong. There are quite a few factors that affect burn rate. Burn rate charts can be very misleading due to those variables.

[GregLaROCHE]

I view burn rate charts as guess-estimates. They can be very useful to an experienced reloader who knows about working up loads. You can’t think you can draw a line across a chart and everything will be equivalent. Powder companies do tests with their powder on many different loads. You can count on the outcome with their data. Using a different powder, from a burn rate chart, you are experimenting and have no guarantee of the outcome. There can be a lot of variables.

[megasupermagnum]

Such a measurement can be made, and probably has. It wouldn't be any good to us reloaders. All we really care about is the velocity a certain weight of powder at a certain pressure gives us. This is called relative burn rate, and there is a mathematical formula for it. That is what those charts are. This is because everything we do revolves around powder weight. Things would be WAY different if our standard was volume. I don't know how this is tested, but I'm guessing it is dependent on the purpose. It wouldn't make any sense to test IMR 4064 at 15,000 psi, the same as it wouldn't make any sense to test IMR 700x at 50,000 psi.

[uscra112]

Yes, it's complicated. Very. Have a look at all the powder parameters that Quickload's software uses to calculate the pressure curve every time you model a load.

[mehavey]

There are more than a half dozen factors that go into what happens after the primer flame hits the powder...


(credit: Quickload manual)

My suggestion is to view "burn rate charts" as colorful rainbows -- NEVER actually load from them

[El Bibliotecario]

This is more than I ever wanted to know about the subject, but ample explanation as to why burning rates cannot be numerically measured. Thank to all.

[GregLaROCHE]

You are listed as a boolit buddy. We don’t know what other experience you have reloading, but it’s always good to be conservative. Start low and work up.

[Land Owner]

Also one brand is not going to test another brands powder.

Whoa there...I'm guessing they do! They may or may not publish, but their employees are JUST as inquisitive as are we. They may have chemists that can solve new formulas to see what makes them tick (you can bet they have done that to their own), whether a new powder might "threaten" their market, undermine their profits, how closely can they replicate it, how can they exploit it with something "similar" but not the same if its characteristics are profound. Know your competitors...

[HWooldridge]

also, just because 2 powders are side by side in the burn chart, does not mean they are close in burn, one could be twice as quick as the other but no other powder fell in between them.

Correct - and your point is often overlooked by beginners. I only use powder charts to rank them in relative order, e.g., Red Dot is generally faster than 2400 - but that’s all. I have reloaded ammo for almost 50 years and still use the manuals, although I have many different editions and often make the various comparisons between them before starting on a new recipe.

I’m also not an inveterate tinkerer - I find a good load and stick to it for years at a time. I have a good friend who’s always playing with formulas and he will change one variable then go shoot a group and record the results. Nothing wrong with either approach - he just likes experimenting more than I do.

[mdi]

IIRC, the charts names have been shortened in recent history. When I started reloading they were called "Relative Burn Rate Charts". No speed attached just the order of burn speed and as it related to other powders...

[Hick]

About the only time you can use a burn rate chart safely is if you happen to have a powder that all charts agree is between two other powders, and if the reloading books give good loads for the outside two powders (above and below). That's why they are correctly titled "relative" burn rate. Even then you need to start at the low end and be careful.

[uscra112]

Going back into the misty deeps of time, when I were a wee lad learning to read my Dad's American Rifleman magazines in the 1950s, experts cautioned strongly against trying to interpolate between powders in a "burn rate chart".

Those who fail to study history are doomed to repeat it. Those who DO study history are doomed to stand by watching helplessly while everyone else repeats it.

[BLAHUT]

I have been forced to fall back to burn rate charts, when powder of xxxx brand was not avabile and one next to it on the chart was>> start %10 or so down on load and work up slowly, saftely.