there is no air in the case with BM2 but I wouldn't call it compressed.
With the 458Win Mag I have decided to only use 480's and 450grn bullets to counter and pressure problems that may occur with bigger bullets.
The 480grn bullets were use with great success in the original 450cal rounds of yesterday.
I do have a few tins of the discontinued 2206 that I will also be using with this round (around 75grns).

Oh, and a slight crimp.