I have been revisiting using heavy bullets in the 450B. Specifically a resized Remington 405 grain 0.458 bullet. I do have a new barrel with a mid-length gas system and my original loads with Lil Gun do not work well with the mid-length system. This bullet/powder combination originally was used in a carbine length gas system. But in the new mid-length the port pressure is to low for reliable functioning. So I moved to slower powders that would boost the port pressure. Currently I am using 10X, as it was a powder I had on hand that fit the criteria.
Using QuickLoad, I settled on a few charge weights that would keep the chamber pressure for the combination between 30K and 36K psi.I shot them a few days ago and ended up with a charge of 36 grains that gave very good accuracy at 100 yards. In fact the accuracy of the new barrel (1/16 twist) versus original barrel (1/24 twist) is greatly improved. Original groups were in the neighborhood of 2 1/2 inches and the new barrel was 1 inch. Also, the mid-length gas system is much easier on the brass.
So getting to my problem, QuickLoad predicts the velocity at 1595 fps. Actual velocity is 1450 fps. Frankly, this is where I want to be with that bullet weight anyway. But it is apparent that after 300 grains in bullet weight, QuickLoad does not predict well. So if the velocity is off, what about the pressure? Any thoughts?