Ballistics discussion on interchanging powders

**darkker** · 09-04-2021, 12:38 AM

For a little deeper dive into the how's, on powders being "the same" based on a burning rate chart, here is some info for anyone interested.

There are a few ways to come up with a burning rate chart, and it's all dumber than you think. The first way is called the Crawford method, from the WWII era. It's a fixed-volume, bomb calorimeter. This determines the powders burn rate factor.
For any burn rate chart that is a "relative burning rate" chat, which most the reloader will see, in fact are.
Those are a combination of "what if's". It assumes the burn rate factor, energy content, and in the magically unstated standard, the progressivity of the powder.

There is actually a wonderful description of this in the Norma manual from 2013. There is a very heavy pressure gun which fires the standard cartridge and charge of powder. Then every other powder is fired in that same cartridge, and bullet. The pressure recorded and assigned a number. Anything with more pressure gets assigned a "faster" burning number; anything lower pressure gets a "slower" burning rate assignment.

Bofors at least at one time, had their standard, as the 308 with 146gr bullet and 43.2gr IMR4350. At some point they won't test anything "faster" as it's simply a bomb. Can you imagine 43.2gr of bullseye in the 308?!🤯🤯

As we all know, volume changes burning rates, especially with the progressive powders. So simply using one standard cartridge, which is almost never disclosed within the industry; Then assigning a relative value across all tested. That really doesn't tell you much at all, about what the powder actually does or can do, in any given situation.
Moreover, manuals essentially never state whether or not they actually pressure tested the data, or if they reprinted supplied data, when that data was tested, what lot numbers were tested, or if the data was simply calculated.

Many of you may remember something like 15 years ago, when Hodgdon suddenly pulled all data from their website for Winchester powder. That was due to contact negotiations. Olin was supposed to supply data for products they don't make, and Hodgdon was too publish and distribute the Winchester brand name. Right/wrong/indifferent, Hodgdon at one point claimed the previously supplied data was so old, they didn't feel comfortable publishing any more. Corporate negotiations being what they are, you may read into that anything you like.

The greater thing to not lose sight of, is that the reloader is the waste market for gunpowder; and standards on blended garbage aren't stated.

Cheers

**charlie b** · 09-04-2021, 09:24 AM

And the surprising part is, over many years and many iterations, some powder reacts almost exactly the same as it did 30 years ago. For example, the 'old' and 'new' IMR3031 and H4895 give the same velocities with the same charges as they did back in the 'old days'. On some other forums there are some more curious folks who have even done pressure traces and found the pressure curves are even within the statistical error of being the same.

And, yes, I understand that some powders are more 'basic' than others. 3031 and 4895 were developed for military contracts so have a pretty large surplus base. I suspect many powders are based on those with modifiers for making them burn a bit faster or slower.

Small arms internal ballistics is one of those areas where there was not a ton of measurements done. Back in the 80's I did some work with the folks in the Army labs (I was still in the Army back them) and we talked about small arms testing, specifically about computer modeling. I was kinda surprised that they had no models and did little analytical work. Almost all of it was statistical based on measured date. It was easier to just batch test stuff. Get a lot of powder, load a bunch of rounds and shoot them. Get measurements and adjust the loads. I suspect the current methods aren't much different given how powder is made.

Larger canons were different since shooting them was a lot more expensive (and required large ranges). They had some pretty detailed models for the internal ballistics on those guns. Unfortunately they did not scale well. The main reason being that the bigger guns used single base powders and modified the grain structure to change the burn rate. The modeling was concentrated more on calculating the surface area of a grain of powder as it burned. And the single base powders (sometimes black powder) had some very predictable pressure/rate curves.

I learned a lot from those guys. Side benefit was getting involved with the folks at Watervliet and seeing them change over their production processes from the old WWII stuff to the modern CNC stuff.

**darkker** · 09-04-2021, 09:00 PM

You remember Ike's warning about the military industrial complex? Long before the 80's the military was doing very little work, they simply paid for things from defense contractors.

In any given application, many of the branded powders in vogue currently, were developed before WWII.
Recently there have been some ingredient changes, but the desired end result never changed. That's part of the reason why it's hilarious to me, that so many people get bent on powder X, and it's "magically extreme" properties. The truth is, unless you are actually measuring with a strain gauge, you really don't know what you think.

For non-testing purposes, vehicle l velocities are the closest approximation for pressure estimation. But it doesn't tell you what the curve is doing.

I'll make an attempt to attach some traces for the point.
https://photos.app.goo.gl/GBwRHFJeWXWEKCzQ8
https://photos.app.goo.gl/rNxNV8fwaaQ6ZnQf8

Look at how wildly different the burning curve is, and the pressures, yet the velocity differences are almost inconsequential.

Cheers

**charlie b** · 09-04-2021, 10:51 PM

Yep, I was part of both sides of the military industrial complex. Add in the halls of congress and you have the triad of power. I did like a lot of the 'real' engineers in the various military labs and arsenals. They frequently were not allowed to do much, but, many were a lot smarter than their contractor counterparts.

Yes, totally agree on pressure measurement. The strain gauge system works well when used properly. It is excellent for comparing load components. I cringe every time someone says their load is OK cause they have no 'signs' of excess pressure.