Gil is absolutely correct. To attain a common velocity two different pressures still have to generate similar energy. One can attain that in a very short distance in front of the chamber. The other, a lower pressure load, stretches out its energy creation further down the barrel. So, if I’m shooting an older gun with thick chambers but tapers quickly I choose a higher pressure load, keeping the stress in the strongest portion of the barrel. If the chamber is marginally thin, as many very early breechloaders were, then I’ll opt for a slower powder to distribute pressures into the bore. Common sense.
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