Powder Burn Rates, Pressure, And Accuracy.

In Blog by Don4 Comments

Hey guys, it’s your favorite Varangian fron Vinland again. Today I might come over all Ben Stein, but what I’ve got to talk about can be very important. Some of you who think you may know it all might even find yourselves learning. Today I’m going to talk about powder burn rates, pressure, and accuracy. Have those who wished to run away yet? Alright for those of you who were kind enough to stay, let’s get rolling.

What brought that on?

I’m glad you asked conveniently written start of paragraph prompt. While perusing somewhere else I saw a topic a person asked about two powders that can on the face appear to just be the same powder with different branding on them. Specifically in this case it was IMR 4350, and Hodgdon H4350. After being told one of the main differences between the two powders, that one has a slightly faster burn rate than the other, the poster responded with this:

Is a slower burn going to be more accurate? I can’t find any H4350 but plenty of IMR 4350 in town right now. As a newer reloader and precision shooter, will I notice a great difference between the two?

Okay so we’re going to have to open up a few cans of worms here in order to address everything. This might get dry for some, but for some of you new guys out there this info has the potential to give you a much deeper understanding of what’s going on inside your gun after you pull the trigger.

Let’s Talk About Powder Burn Rates

Not everyone may know this, but not all powders burn the same. Some burn fast, while others burn slow and that’s all fine and good. Here’s the powder burn rate chart.

What does that have to do with anything? Relatively slower burning powders tend to require more powder for a load, and can produce higher velocities. Now there are a number of reasons why we would want this. This is due to chamber pressure being the limiting factor, with quite a number of calibers coming in and around 60,000 PSI. To put that in perspective, that’s like having the same force as two Caterpillar D7 bulldozers on every square inch. Anyways if we were to graph data of pressure vs vs distance traveled in a barrel we would notice that the faster burning powder hits a much higher peak at a shorter distance than the slow burning powder. This is actually deceiving if you don’t think about it too much because someone could potentially say “Well that’s the one you want, it hits a higher pressure.” Well not exactly. For handguns this might be the case, but we’re dealing with rifles here. With all powders after peak pressure, as the length of the barrel increases the pressure decreases. With fast burning powders this decrease after the peak is much more rapid, and slow burning powders are much more gradual.

If one has some knowledge of physics you might have already heard of Boyle’s Law. For this we’re going to assume that the temperature of the combustion products are at the same temperature. A quantity of powder can only produce a limited quantity of gas, the case, barrel, and base of the bullet where the gasses are contained has a variable volume. Since we have a fixed quantity of gas, as the volume increases, the pressure decreases. The mathematical equation P1V1 = P2V2, or the initial pressure multiplied by the initial volume equals the end pressure multiplied by the end volume.  This is assuming that the temperature of the gas is constant. Have a look at the following picture.

Powder Burn Rates

As you can see from the graphs, the pressure curve for a fast powder peaks much earlier. On the other hand for a slower powder it peaks, but then decreases at a much slower rate. It maintains a higher average pressure over a longer period of time, and also has a much higher pressure at the muzzle. Why is this? Because there’s still powder burning in the barrel as the projectile is traveling down the barrel. This means that there is a greater quantity of gas being produced. Remember Boyle’s Law? Another thing to consider is that slower burn rate powders tend to be larger in volume. This can actually be a safety factor as well. If a case has a volume of 70 grains, and your load takes up 66 grains of powder, then it is extremely hard to over charge or double charge.

One final benefit of a slower burning powder is that as the velocity increase is far more predictable as the amount of powder increases. For example, recently I was doing some load development with a buddy on his 7mm Rem Mag. Despite some problems, the data I do have is able to tell a story, and can be used to potentially predict the velocity of the next increment of powder. I’ve calculated the linear regression of the data, and because of that I’m potentially able to calculate what the velocity would be at the next increment of powder.

Here’s a graph of the data. The X-axis is the grains for the load, and the Y-axis is the FPS of the load.

What’s the Difference Between IMR 4350 and H4350 Then?

Aside from a small variation in powder burn rates and branding? Well one of the most important differences is H4350 is one of Hodgdon’s Extreme powders. What does this mean? Well Hodgdon’s extreme powders, as well as the IMR Enduron series, are meant to be temperature insensitive. As the temperature decreases, velocity decreases, and the converse is also true. So what this means is that as temperature changes IMR 4350 will have a greater change in velocity than H4350. For us that is the most important detail, but other quality of life things like shorter grains for easier metering is nice too. For me though, temperature insensitivity is a big driver considering where I live.

H4350. Also known as "Unicorn's Blood"

H4350. Also known as “Unicorn’s Blood”

 What About Accuracy?

Accuracy and powders. Here’s the thing, all guns are unique. You can put two of the exact same guns next to each other, even consecutive serial numbers, and they can both like very different loads. What’s accurate for one gun does not mean that it’s going to perform the same in another. Tiny differences can actually end up making all the difference. The key is to find the powder and bullet combination that works best for you. Most importantly though I want to stress to everyone that they should remain within the maximum loads published for whatever caliber you are loading. Stay safe out there everyone.

Don is a Minnesota college student working his way through school as a firearms coatings specialist. An avid shooter with a love for just about all things gun related, gladly sharing his somewhat unique experiences with anyone who will listen. If you have any questions for me, email us!

Comments

  1. This is Chris Hodgdon’s response when I asked him if the three different 4350s were in fact the same formulation- internet folklure…

    “No sir-all three versions of 4350 are manufactured in different continents.

    Generally IMR4350 is faster burning than the H version(depends on how it burns in a certain cartridge). All three are NOT interchangeable”

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