...Steve, another thing I noticed in the Hodgdon data you refer to:

for the 148 HBWC, their listed starting speed differs quite a bit: 771 fps for TG and 869 for W231, a difference of 98 fps or 13% in speed.

for their 158gr data, the listed speed for TG is 815 fps versus 782 for W231, or a difference of 4%, much less delta. So it could explain why we see a bigger difference in the case of 148 HBWC.

This explanation is probably better than the one I gave in my previous post

Regards,
Guy -