John:

The reason that loading for the British black powder express rifles like your .450 3 1/4" BPE is a bit different today has to do with the difference between the English 19th century black and modern black. As Mick stated, Victorian era black, such as the Curtis & Harvey's Number 6 that these cartridges were normally loaded with, was a finer granualation than that commonly used in BP rifles today. There is more to it than that though - finer grain modern black behaves differently at higher pressures.

GOEX FFFg usually comes quite close to duplicating old black in low pressure, non-express cartridges. However, at the higher pressures of the BPEs (your .450 with 120 grains black & 270 grain bullet at 1975 fps is rated at 11 tons = 24,640 cup) the GOEX is 15 - 20% less efficient than the old black. The result is that you can't get enough of the modern black into the case to duplicate the original velocity with the same weight bullet. As Mac noted, with your single this is no problem, except that it won't shoot to the elevation that your sight leaves are filed for. This is also why the BPE doubles can be so difficult to get to regulate with modern black - the inadequate velocity increases barrel time causing the barrels to shoot wide. As alluded to in the Seyfried quote that Mac posted, the IMR 4198 "nitro for black" conversion cures this problem with no increase in pressure - which is why it is so widely used for the BPEs today, and why Mac recommended it.
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