Unclenick says:
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So, let’s make an example. You load a .308 in a Winchester case, with the 175 grain MatchKing jammed 0.010” into the lands and find 41.4 grains of IMR4064 gives you best accuracy at a velocity of 2560 fps. You next seat a bullet 0.160” deeper, and find you now need 42.8 grains of 4064 to get to 2560 fps, matching the velocity of the first load. The difference is 1.5 grains. You multiply 1.5 grains by 1.4, giving you 2.1 grains, and add that to 41.4 grains. You now have 43.5 grains. This will be a little faster than the original velocity, but it should be about right to produce the same barrel time as the original load, so it should already be pretty well tuned.
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I wouldn't bet on barrel time being the same for different charge weights of the same powder under the same bullet with the same neck tension. Expecially when you also state:
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But you still have another issue, and that is that as the pressure drops the velocity drops and the barrel time increases, and that can move you off a sweet spot for powder charge or move you onto one.
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It all sounds contradictory to me. Wouldn't barrel time decrease with higher muzzle velocity?
What do you mean by a "sweet spot?" Something with muzzle velocity or barrel whip?