Ballistic stability, just what is involved in designing a ballisticly stable round?

As some of you know, I’m working on my associates degree in firearms technology, and part of that degree deals with ballistics. When I came in to this I thought I had an understanding of ballistics as I am a reloader. However, the more I head down this rabbit hole, the more I realize I didn’t know nearly as much as I thought I knew. Sure, I know about bullets, primers, powder, etc. I also knew all of these things have an effect on how the bullet travels, know as external ballistics. But what I found I knew very little about was internal ballistics. Or, what happens from primer strike to exit of the bullet at the barrel.

I just finished up an article for Underwood ammunition about 10mm rounds, However, at the time I published it, I didn’t notice a error I had on my ballistic calculations. I input the temp, pressure, caliber, all the good stuff. But totally missed the bullet length. I finally noticed last night. No big deal, it’s not like 0.59 and 1.25 is going to make much difference. So this morning I made the correction and I’m like, crap, that made a huge difference. I quickly redid all the calculations and made the change to the article.

So I did a little experiment to give all of you an idea, just how much of a difference does a little bit make.

This image is a 10mm bullet, I have all the settings in. I have bullet length set to 1.0" Yes, that’s a really long bullet for 10mm, a normal bullet would be around 0.6" But this is to make a point. The stability of the bullet is in the green, a bit low, but still considered ballisticly stable.

So now, let’s increase the length of the bullet a mere 0.25" to 1.25" which is where I had the setting.

Boom, just like that it drops to extremely unstable. The length of the bullet was the only veritable that was changed. The amount of math that goes in to designing high grade custom ammunition is mind boggling. And I’m only talking about the first few inches of the bullets travels as it heads to its target. Anyone that can follow simple instructions can load a round. But to design one, that is several orders of magnitude harder.


There is a really great place with all kinds of customizable data calculators you and others can use.
Plug in your data and look at the results.


Nice information there. I use because it also takes in to account temp, pressure, and even a plastic tip on the bullet.


Fun toys! Time to play with some slug loads!