The CUP pressure changes with bullet weight/powder/firearm. Some loads reach higher speeds without exceeding this pressure, usually higher speeds with lighter bullets. My reason for AA#9 is due to my love of Hornady. I can achieve the highest speeds with AA#9 and Hornady without exceeding the recommended CUP pressure. Everybody has their favorites, but I have not found a faster, hard hitting, more accurate, combination
for my gun than my big three: Hornady bullets, AA#9, and Hornady reloading manuals. I've shot deer with AA#9/XTP's and knocked them flat their Heinekens.
This is not the best combo for all. Find whats best for you and your gun by practice and sample. Keep experimenting with recommended safe loads and you will find your favorite . My Redhawk is more accurate with higher speeds and heavy bullets, but when it comes to plinking I use: 700X, Red Dot, Green Dot, and hard cast bullets (more bang for your buck with faster burning powders
).