Here's a little note from Brian Litz. Thought you might be interested.
"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
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When applied by competent people with the right intent, common sense goes a long way.
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