Help me understand burn rate

Gstew1930

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Jun 3, 2020
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151
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Mont Belvieu
I’ve been reloading for about 10 years now, I still can’t quite get my head around why certain powders work great for a certain cartridge but don’t work for another cartridge of similar case capacity. I’ll give an example, H4350 in 6.5 creed is the go to powder for most people. But in 308 win it’s considered to slow, even though the 308 has a slightly higher case capacity. I know there are some really intelligent guys on the forum so help me understand why this is.
 

del2les

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Oct 24, 2007
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838
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South Central, CO
Expansion ratio is the case volume plus bore volume, divided by case volume.

Example: the case holds 7.3 cubic X and the bore holds 3.1 cubic X. So the expansion ratio for that round in that gun is 10.4/7.3 or 1.42. That would be a very low expansion ratio, indicating use of an extremely slow powder.

The general rule on expansion rule is "lower is slower" regarding powder selection. But you can't use it to pick a single best powder, because powders aren't rated by a universal burn speed measurement. You can get only a general idea or "kinda slow" versus "kinda fast" and must experiment after that.
 

Gstew1930

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Jun 3, 2020
Messages
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Mont Belvieu
I was wondering if bore size was more of a factor. I appreciate all the info guys. This has been bugging me for a while lol
 

rammac

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Oct 28, 2010
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SW Montana
The basic problem of internal ballistics is to balance the production of hot combustion gasses against obtaining the desired bullet muzzle velocity and group size.

In general, the shape and the chemical composition of powder controls how the powder burns. Burn rates aren't specific numbers, they are relative to other powders. In other words, when you choose a powder with a certain burn rate, you are simply saying that the powder burns faster or slower than some other powder.

Powder composition – single base, double, base, etc.
Powder definition of configuration – progressive, degressive, neutral.

Pressure/distance curve

1607544919504.png


Burn rate is directly related to the rising portion of the pressure/distance (or time) curve. The amount of surface area of each kernel of powder controls the burn rate.

A higher burn rate indicates that the pressure will rise quicker but it doesn't tell the whole story about pressure and velocity.

Progressiveness is the definition of the configuration of the powder and it controls the tail of the pressure/distance curve.

Progressiveness is a measure of how much hot gas is produced as the powder burns. A more progressive powder produces more hot gas as the mass of powder burns away. A degressive powder produces less hot gas as the mass of powder burns away. A neutral powder produces the hot gas as the mass of powder burns away.

A progressive powder results in a lower peak pressure that is later in time and a higher muzzle pressure compared to either a neutral or a degressive powders.

So changing;
Amount of surface area (more or less powder or the shape of the powder granules)​
Powder configurations (progressiveness)​

...will result in a variety of combinations of pressure peak locations and muzzle pressures while producing the same muzzle velocity (assuming that you use the same bullet and powder charge).

1607545005665.png


Grain size controls surface area and changes in surface area produce changes in the pressure curve. A large grained powder (assuming the same amount of powder being used) will be more progressive and it will produce the max pressure later in the burn time curve and the muzzle pressure will be higher.

Like all things, there are trade-offs when you choose a powder. Slower, more progressive powders produce higher energy levels (because the pressure curve stays higher longer in time) and higher energy levels produce higher velocities. Unfortunately, slower powders are less efficient than faster powders and less efficient powders produce larger velocity variations and larger velocity variations produce bigger groups on target. So the goal would be to find the slowest, progressive powder that produces the velocity and group size that you want and that will fill to the highest density that will fit in your case.
 
Last edited:

MagnumManiac

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Feb 25, 2008
Messages
3,143
There is a major flaw in everyone’s thinking when it comes to burn rate.
The simple fact of the matter is that burn rate, and how it is measured, are flawed, because case shape, size and volume as well as bore size all correlate to CHANGE the burn rate. Burn rate is NOT CONSTANT.
Some powders swap positions in burn and behave differently. Example, in the 338WM RE 22 SHOULD give higher velocities than RE 19, but due to Expansion Ratio, RE 19 with 225gr bullets is BEHAVING slower and producing higher velocity. A similar occurrence happens with the same powders in 30-06.
It was also noted some time ago that straight walled cases slow down the burn, so medium burn rate powders ACTUALLY behave much slower in these and provide the highest velocities.
A Calorimeter bomb is used to attain burn rates...why are they flawed?
The reason they are flawed, and I discussed this with the techs at ADI (makers of all Extreme Hodgdon powder), is because there are ZERO set standards in the industry. All manufacturers use RQ numbers (Relative Quickness) to list their powder from fastest to slowest.
This is the kicker, being there is no standard, each powder maker chooses a powder in the middle, this may be IMR4895 for IMR, H4895 for Hodgdon, and given a RQ of 100. Then all other powder is measured in the same Calorimeter bomb and given a higher/lower number depending on whether is faster or slower than this.
What is measured in a Calorimeter bomb?
Absolute pressure (AMP), Time & Heat.
Now none of this correlates to a cartridge case or firearm. In fact the burn is completely different, as there is no barrel exit, it is just a container with measuring equipment attached and a confined amount of powder is detonated inside it.
When I worked at ADI, these tests, which are very expensive, were only undertaken ONCE on a new batch of powder to see how it stacked up to previous batches.
To be sold as Hxxxx, it HAD to be within 3% of ALL previous batches in Calorimeter results, which are different to actual pressures tested in a firearm.
If it didn’t match this 3% variance, it became a bulk powder and sold to ammo manufacturers.
Now, I have a pressure trace which is a piezo strain gauge attached to the barrel above the chamber, it gives raw pressure data. I have seen swings of 10% with different batches of powder, especially with H4350, which is beyond a 3% variance, but this 10% difference DOES NOT occur in every cartridge that powder is used in...why?
BURN RATE IS NOT CONSTANT IS WHY.

Sorry for the long post, but this needs to be digested and understood if you want to understand how smokeless powder behaves. There is more to it, such as Progressive powder, Digressive powder and Regressive powder.

Cheers.
 

Mike Matteson

Well-Known Member
Joined
Jun 26, 2017
Messages
1,048
The basic problem of internal ballistics is to balance the production of hot combustion gasses against obtaining the desired bullet muzzle velocity and group size.

In general, the shape and the chemical composition of powder controls how the powder burns. Burn rates aren't specific numbers, they are relative to other powders. In other words, when you choose a powder with a certain burn rate, you are simply saying that the powder burns faster or slower than some other powder.

Powder composition – single base, double, base, etc.
Powder definition of configuration – progressive, degressive, neutral.

Pressure/distance curve

View attachment 231672

Burn rate is directly related to the rising portion of the pressure/distance (or time) curve. The amount of surface area of each kernel of powder controls the burn rate.

A higher burn rate indicates that the pressure will rise quicker but it doesn't tell the whole story about pressure and velocity.

Progressiveness is the definition of the configuration of the powder and it controls the tail of the pressure/distance curve.

Progressiveness is a measure of how much hot gas is produced as the powder burns. A more progressive powder produces more hot gas as the mass of powder burns away. A degressive powder produces less hot gas as the mass of powder burns away. A neutral powder produces the hot gas as the mass of powder burns away.

A progressive powder results in a lower peak pressure that is later in time and a higher muzzle pressure compared to either a neutral or a degressive powders.

So changing;
Amount of surface area (more or less powder or the shape of the powder granules)​
Powder configurations (progressiveness)​

...will result in a variety of combinations of pressure peak locations and muzzle pressures while producing the same muzzle velocity (assuming that you use the same bullet and powder charge).

View attachment 231673

Grain size controls surface area and changes in surface area produce changes in the pressure curve. A large grained powder (assuming the same amount of powder being used) will be more progressive and it will produce the max pressure later in the burn time curve and the muzzle pressure will be higher.

Like all things, there are trade-offs when you choose a powder. Slower, more progressive powders produce higher energy levels (because the pressure curve stays higher longer in time) and higher energy levels produce higher velocities. Unfortunately, slower powders are less efficient than faster powders and less efficient powders produce larger velocity variations and larger velocity variations produce bigger groups on target. So the goal would be to find the slowest, progressive powder that produces the velocity and group size that you want and that will fill to the highest density that will fit in your case.
Thanks for the input!!!!
 

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