That charge range is right for the 168 grain match bullet, which is 1.215" long, same as the Sierra 168 grain MatchKing. If you seat the A-max (1.265" long) to the same COL, it will be 0.050" deeper in the case. QuickLOAD thinks that will raise the peak pressure about 6% and the velocity about 30 fps. That's probably not exactly correct because the A-max has a longer ogive and will therefore have a bigger jump with longer gas bypass time getting to the throat, tending to mitigate the pressure difference some. and may well want to be on the low end of the load range. But if I ignore that, it seems to want about 0.7 grains less IMR 4064 to get to the same peak pressure and about 0.5 grains less to get to the same velocity, but doing so with not quite 0.7% higher peak pressure (a negligible amount in the grand scheme of things).
Tony,
Note that Mississippi's half grain increase is to match velocity and not peak pressure. If you match peak pressure in the larger case, then velocity will be higher, as I indicated. Pressure peaks occurs when the bullet is only an inch or two into its journey down the tube. As it continues down the rest of the barrel beyond that point, pressure drops. In general, the more powder you used to reach the peak pressure the more gas is made so that post-peak pressure holds up better. That's what you get with a slower burning powder. The bullet's energy at the muzzle, which is proportional to the square of its velocity, depends on the overall average force due to pressure it experiences behind it as it travels from case to muzzle. Peak pressure is commonly on the order of 5 times higher than muzzle pressure, but the ratio gets smaller as the powder burn rate gets slower. All this is to explain why the peak pressure does not have direct correspondence to velocity. Changing powder burn rate changes the peak pressure needed to get a certain velocity.
Here's the general rule: The larger the case, the more powder it takes to reach a given velocity, but the lower the peak pressure at that velocity. Conversely, achieving the same velocity from a smaller capacity case requires less powder, but produces a higher peak pressure and lower muzzle pressure to achieve the same average pressure. That's all about how much total gas is made and how well it holds up pressure in the barrel beyond the peak.
As a result of the above, if you develop a load in a tight case, and you have a good chronograph and know how to use it and do all the testing in the same light and temperature and bore fouling conditions, you can then adjust that charge to achieve matching velocity in your larger cases. But you can't count on going the other way! That is, you cannot take a load developed in a larger case and reduce it to a matching velocity in a smaller case without increasing peak pressure to some value higher than the one you worked up to in the larger case.
YOU ARE CORRECT about measuring case water capacity in an as-fired and not resized case, with this caveat: Cartridges that peak at pressures of about 30,000 psi or above expand enough soon enough to stick to the chamber walls and stretch as the case head reaches the breech. This expands the powder burning space with the result that the as-fired, expanded volume is what influences peak pressure. Cartridges that peak below that pressure, however, tend to back up in the chamber without stretching, so the resized volume then becomes a better predictor of pressure. For the .308/7.62, the expanded as-fired volume is what you actually want to use.
