If you look at the old Lyman BP manual, the one with Rd Yard's pressure data, it is pobvious that FFFg can produce velocities as high as FFg will, but at somewhat higher pressures, like 12,000 to 15, 000 PSI versus 7,000 to 10,000 PSI. This increase is noticeable, but not enough of a jump to damage any reasonably made gun!! Most BP cartridge rifles are built to be safe at 25,000 PSI or higher, like the Sharps or Rem. Rolling Block.

Keep yore powder dry!!