I loaded up some 300 gr loads (64 and 68 grains of IMR 4064 respectively) -- no issues. When I loaded my 500 grain bullets over 69 grains of 4064, the powder nearly filled the case and, in order to fit the cartridges in the magazine box(?), the powder was very compressed.

Any concerns? Would buying/using a brass drop tube do much good?

change powder.. you have discovered the reason for the 458 lott and the failure of the 458 winmag
h335 and rel 7

Thanks Jeff. I'll look for those powders. In the interim, I have 4 lbs of 4064 to burn up -- any issues with compression of the powder?