RH, you lost me on the 72 fps change in MV. How did you get that? I see that 6% of 1200 fps is 72 fps. However, you can't equate a change in a point pressure to MV (you must integrate the pressure-displacement curve). Remember, loads of slower burning powder can generate equal MV with lower chamber pressure (but higher barrel pressure such that the faster and slower powder have equal area under the pressure-displacement curve).