The issue that was brought up with 2Piper has to do with the concept of “work” on the projectile. In order to have the same velocity, the work done in both cases must be the same. Mechanical work is the amount of energy transferred by a force acting through a distance so in this instance it is the pressure force over the distance down the barrel.
A fast burning powder has a sharper pressure build closer to the chamber but decreases more quickly than a slower burning powder. If you were to plot the pressure profile down the barrel, the area under the curve represents the work done to the load. If the velocities are the same, then the work is the same. Given that, the 7000 psi load will have a rapid rise and decrease in pressure whereas the lower pressure load will maintain the pressure longer.
At some point in the barrel, the pressure curve will drop below the 4300 psi curve. So although the peak pressure at the chamber is higher with one load, the other will have a higher barrel pressure for some portion of the time.
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