Great guidance from gunner500.
This thread got me to look up the Graeme Wright data on the .450 x 3-1/4" BP Express.
I was surprised to see that his pressure tested data for duplexed loads (SR4759 + GOEX) was only 6.7 to 8.1 tpsi,
lower pressures than the old standard ammo with 19th-century BP, 11.0 to 12.0 tpsi.
Whether those duplexed loads would shoot to regulation is another question.
The lighter powder mass with duplexed loads would raise the muzzles less and the rifle might shoot low and crossed
without the full mass ejecta of a straight BP load and its greater recoil.
From another thread, here are the excerpts for book review purposes:
That is an excellent book. The general chapters cover many aspects of the care and feeding of double rifles and single shots firing same ammo.
Very interesting how 20 grains of SR4759 plus 60 grains of BP is equivalent to 120 grains of BP with similar pressures for both loads.
Very low pressure with .458-caliber cast bullets of 320 to 358 grains and a 350-gr jacketed soft nose.
Only 6.7 to 8.1 tpsi, whereas the historical pressures for similar bullets, BP charges, barrel lengths, and velocities were 11.0 to 12.0 tpsi.
The imperial tonne or "long ton" is 2240 lbs. 12.0 tpsi = 26,880 psi.
The duplexed 358-gr cast bullet at 1700 fps has a pressure of 7.2 tpsi = 16,128 psi.
1 grain of the smokeless "pistol powder" substitutes for 3 grains of BP.
Cork wads 3/8" thick sit between bullet and powder.
Must be no or very little compression of powder.
By total weight of propellants, the duplexed BP load is 25% smokeless.
Seems like a high percentage of smokeless by weight might be helpful with light bullets ?
Kirton recommended a load of 5 grains SR4759 plus 85 grains of GOEX FFg in the .461 Gibbs with 540-gr bullet.
That is a bit over 5% smokeless by weight, worked well with the heavier bullet, for about 100-gr equivalency to BP.
The old Curtis & Harvey No. 6 BP must have given higher pressures (and velocities ? ) than GOEX.
GOEX is apparently easily and safely spiffed up with SR4759, eliminates most of the BP fouling,
and gives lower pressure than the old BP loads !