Duplex Loads and Double BP Rifles

[trapperP]

I have been reading a bit re the older blackpowder cartridges, especially the bigger ones such as the 45/70 and up. Couple of questions I would like to know more about.
1] Ran across a reference to a group of loads using 5-6 Grs of a powder referred to as "PB" and then a case full of black powder compressed to allow room to seat the bullet.
What was the exact purpose of the smokeless powder under the black, better ignition? That is the only reason I was able to surmise.
2] Colt made some double barreled rifles in the late 1880's period that had a RH twist in one barrel and LH twist in the other. What reason would they have for this? Try as I might I cannot fathom any good reason for doing this - anybody?

[Don McDowell]

Generally speaking the smokeless charge in a duplex load should be no more than 10% of the total bp load, and the bp should not be compressed when using duplex. Duplex loads do burn cleaner, leave little fouling, but also raise pressures and velocity.
Just much better/safer to use either all black or all smokeless.

[NickSS]

Duplex loads got started about the same time smokeless powder was invented. The prime purpose was to make it cleaner burning and leave less fouling in the bore. I have read articles on this process dated in the 1890s. I have used duplex loads when I first got started shooting black powder in long range bulls eye matches. It was allowed and does keep fouling down. Then I started shooting straight black and found that I was getting better accuracy and the bore was easier to clean at the end of the day. So today I shoot straight black and use a blow tube to keep fouling moist during a string of shots. I have fired as many as 50 rounds without wiping my bore but in competition I wipe the bore between strings. Some competitions only allow straight black powder. For 900, 1000 and 1200 yard matches they allow up to 10% duplex loads by NRA rules (unless they have changed the rules recently). The reason it to get the velocity up especially in the 45-70. A smokeless powder like 4198 is about equivalent to 3 gr of FFG in a duplex load. So a charge of 7 gr is equal to 21 gr of black. Add that to 64 gr of FFG and you have a load in a 45-70 case that is about equal to a 45-90 using straight black of about 100 +fps more velocity.