chamber pressure & muzzle energy question for Bryan

[Park ranger]

Bryan, in your book, and a few other online post, I believe you stated that if two different weight bullets were being compared in an apples to apples test, you use muzzle energy as the indication the chamber pressure was the same. IE, equal muzzle energy = chamber pressure.

Two questions. If I understand this correctly. ..

1.With that in mind, is that an ideal way for me to work up an untested load, say from my 155 to a 185 juggernaut in my palma rifle?

2 . I noticed that when I went from lapua brass to winchester I had to add about 1/2 grain more powder to get the same velocity. When my velocity is equal , same bullet, is it fair to say chamber pressure is the same despite difference in case capacity and powder charge?

Thanks Bryan, I really like having the follow up opportunity from the text books.

Roy

 

[Bryan Litz Ballistics]

Roy,

The 'equal chamber pressure means equal KE' scaling of MV is a general rule, not exact. Certain practical variables in the real world will cause the relation to not hold exactly. Those variables are things like:
1. When you change bullet weight/length in the same chamber, you're affecting the internal case capacity. In order for the relation to hold, when going to a longer/heavier bullet, you would have to throat the chamber out to allow the same internal case capacity.
2. Bearing surface length. Longer/heavier bullets tend to have longer bearing surfaces which take more energy to engrave which affects the energy balance.
3. There are subtleties to the way powder burns in reaction to the time vs. pressure which will affect things too.

Having said those things, the general rule of thumb is still a good rough estimate. In the case of going from 155's to 185's in the Palma gun, let's say you were getting 3000 fps with the 155's. Your expected velocity with the 185's would be: 3000*sqrt(155/185) = 2746. Now if you load the 185's in the same chamber as the 155's, you'll probably fall short of the 2746.

Regarding your Winchester vs. Lapua brass, I would say that when you've matched velocities in the same barrel, that you've matched pressures. Although it's splitting hairs meaning I don't think the difference would be large.

-Bryan

 

[Sheldon N]

When my velocity is equal , same bullet, is it fair to say chamber pressure is the same despite difference in case capacity and powder charge?

Splitting hairs here, but wouldn't the pressure in the Win brass be slightly less than the pressure in the Lapua brass at equal velocities?

Bigger case capacity @ same pressure = higher velocity?

 

[Park ranger]

Thank you Bryan. The general rule was what I was looking for. I just wanted to make sure I understood the concept.

 

[usmc45]

Very cool little formula and fun to play around with. Thanks Bryan. Im still new to all this so i am assuming that this works in the other direction as well, say going from his 180 down to 155. Also, all things being equal would going from the 155 to the 180 increase chamber pressures? Thanks for all the info.