What is 'burn rate' measuring ?

[SteveW-II]

I understand the concept of powder 'speed', but what is the often quoted 'burn rate' of a powder a measure of ?

Why is no standard metric quoted (grains per nano-second) ?

I guess I am looking for an answer that says something like :

a) How quickly (fractions of a second) an amount (grams, grains, ounces) of powder is totally consumed (burnt away, oxidized by fire) at atmospheric pressure (1 bar, 14.7 PSI etc).

or

b) The time required for the ignited powder to reach a range of pressure at which there is a liner correlation between 'x' amount of powder consumed and 'y' volume of gas produced..

Again, I understand that as the pressure rises in a confined space, so that pressure reacts with the burning powder to change the burn rate. I understand that powders have 'working ranges' at which burning is complete. It's clearly an equation of powder volume (amount), pressure and time. I just don't understand why it doesn't appear to be well defined.

S.

 

[rcmodel]

As I understand it, laboratory testing is done using a "closed bomb" test to determine burn speed.

It can't be defined in a gun, or all guns, because each caliber / cartridge has a different internal volume, bore size, etc..

You could have a certain "burn rate" powder used in a small bore high-velocity caliber turn up completely different in a big-bore small capacity caliber.

It is rare to even find two different burn rate charts that agree with each other totally.

rc

 

[ranger335v]

"I understand that as the pressure rises in a confined space, so that pressure reacts with the burning powder to change the burn rate. I understand that powders have 'working ranges' at which burning is complete. It's clearly an equation of powder volume (amount), pressure and time. I just don't understand why it doesn't appear to be well defined."

First half of your statement answers the second half. It isn't well defined because there are too many variables no one understands.

It all happens pretty fast in a chamber and it's hard to see in there!

 

[WNTFW]

Steve,
I think it is an excellent question. I feel like my understanding of burn rate is a weak link in my reloading. Then again I go by published loads so I'm not sure how knowing more would affect my choices. What powder is available makes a difference also.

I have read or heard somewhere that "Burn Rate" is not the correct term and "Relative Quickness" is. So why are they called burn rate charts? Or what is the quickness relative to - probably each other.

Burn rates can seem a little counter intuitive to me. I guess all the relationships between bore, case & bullet weights are too much for me too comprehend correctly every time.

Somedays I really like to stick to my .22lr. Not much to control or think about other than just shooting and wonder how much the bullet drop will vary.

Later,
WNTFW

 

[SteveW-II]

> It isn't well defined because there are too many variables no one understands.

I find it hard to agree with that. I think most applied physics has been mastered by computer modeling, so I believe that if we took the trouble, it could be well understood.

Even if there are too many variables, why is there no standard unit of measurement, such as grains per second (or much smaller fractions of).

The way things are right now, the published tables of burn rates are meaningless. I mean, what can you use them for ? When I first looked at them I though it might give a guide of powder interchangeability, but without a meaningful and consistent metric, you have no idea of the size of the 'gap' in the table between powders.

 

[ranger335v]

"...you have no idea of the size of the 'gap' in the table between powders"

Yep.

Useful computer models must have hard data to establish reference points for control. Powder burn rates, and "global warming", don't, far too many unknown variables. So any supposed computer models are only colorful, fanciful figments of the programmer's imaginations, just WAGs (Wild Azz Guesses) and not "science" at all.

RC nails down 'burn rate' in post #2.

 

[USSR]

It's a relative thing. A good part of reloading is more art than science.

Don

 

[ants]

OP asked about a common or standardized unit of measure.

As defined by SAAMI, the Sporting Arms and Ammunition Manufacturer's Institute:

POWDER BURNING RATE
The speed with which a propellant burns. It is affected by both physical and chemical characteristics, as well as conditions under which it is burned.

Standards for comparison used in each industry are generally set by national or international organizations such as ANSI, ASTM, SAE, and others. SAAMI is an Accredited Standards Developer for the American National Standards Institute (ANSI), and would be assigned the task of setting standards for testing (and thus the unit of measure) for gunpowder burning rates. Unfortunately, it does not appear that ANSI is interested in establishing a standard for the testing of powder burning rates, nor does it appear to be imminent. So, as described many times above, we're on our own. There is no standard unit of measure for comparison.

For a better chart that uses a graphic representation for powder burning rates among manufacturers, go to the VihtaVuori web site and download the current reloaders guide. Their tabular chart is organized in a superior format.

 

[cliffy]

Pressure Build-Up

FAST powders designed for pistols is NOT designed for Rifle usage. As a .243 Winchester shooter, I'd no sooner shoot Alliant 2400 rifle powder in my rifle than Unique powder. One MUST find the correct burn rate to safely load any bullet. I fire Hodgdon H4350 FAST rifle powder through Alliant Reloder 22 SLOW powder as my range of burn speeds for BEST PERFORMANCE. TOO SLOW: WEAK, while TOO FAST can be DANGEROUS. Burn Rates are not merely charts; They put you within the right ballpark. cliffy

 

[jeepmor]

It isn't well defined because there are too many variables no one understands.

Bzzz, wrong answer. It is very well understood, but the manufacturers don't share that info for obvious liability reasons.

There's a system you can mount on your chamber and measure the impulse of a round. When you have round of "known" pressure, you can set your baseline and measure different powders impulse curves, pressure levels and so forth. It's basically just a strain gauge that connects to a high speed data acquisition unit and a laptop. Probably the best "practical" method for a reloader that exists.

Here's a link
http://www.shootingsoftware.com/ftp/Instrumenting%20your%20rifle.pdf

 

[ArchAngelCD]

SteveW-II,
Most of the "Burn Rate Charts" are really "Relative Burn Rate Charts." The charts are meant to be a guide in aiding in the choice of a powder, not to be exact or precise for every possible application. The level of exactness you are looking for could only be put to use by a Ballistic Engineer but then again, if you were a Ballistic Engineer you would know the formula you are asking about. The charts are just an aid...

 

[USSR]

Most of the "Burn Rate Charts" are really "Relative Burn Rate Charts." The charts are meant to be a guide in aiding in the choice of a powder, not to be exact or precise for every possible application.

Yep. Different charts list different powders in a different place in the chart relative to each other. For example, some list H4895 as being slightly faster than IMR4895, while some have it the other way around. Still, it puts you in the ballpark regarding the suitability of certain powders.

Don

 

[ArchAngelCD]

Still, it puts you in the ballpark regarding the suitability of certain powders.

Don

My point exactly, I agree...

 

[Gewehr98]

Don't get too hung up on the "burn rate" thing.

It won't really help you in load development. Leave that to the published load manuals, and work up your loads from starting amounts paying attention to pressure signs and accuracy.

Never say never, either. Case in point:

FAST powders designed for pistols is NOT designed for Rifle usage. As a .243 Winchester shooter, I'd no sooner shoot Alliant 2400 rifle powder in my rifle than Unique powder. One MUST find the correct burn rate to safely load any bullet.

The above is a blanket statement. I run Alliant 2400 in my rifles, and it works damned good in my cast bullet loads for .30-30, 7-30 Waters, .38-55, and .45-70. There are even published jacketed bullet loads for the .45-70 using Alliant 2400, so going by burn rate or calling it a dedicated pistol powder is somewhat disingenuous. There are dedicated shotgun powders that have garnered wide acclaim for use in centerfire autoloading pistol rounds, like Clays, WST, WSF, you name it. So be very careful with assigning blanket assumptions.

It's good to have a chart of comparative burn rate characteristics, if only to have a ballpark estimate of what family of powders you're using for a given application. But don't let that be your be-all, end-all of reloading data.

 

[redneck2]

If you haven't read the above post by G98, I'd suggest going back and reading it....two or three times

Now, one thing that I'd add. "pressure signs" can be very misleading and get you into trouble depending on the particular round you're loading. You need to be WELL aware of the target PSI that you're looking for. Sub normal for many rifle rounds would destroy many pistols.

Some pistols operate at the 15k. You will not see flattened primers until way past that.

Some advanced reloaders want to use case head expansion. This has been shown to also be inaccurate and unreliable depending upon the particular firearm.

Examples of this are the lowered recommendations of many of the manufacturers. While the radical reloaders think this is due to over reaction to our lawsuit happy society, in many cases it's the realization that old loads developed without modern test equipment were too strong.