Thanks to badgeredd for an e-mail asking me to report in on this thread.
Fellas, this isn't a big deal and it has been done since smokeless powder was invented. It is NOT anything new, nor is it dangerous to do or I would not do it.
Generally you can safely load up to 15 percent of your total powder charge with smokeless powder. I tend to stay at about 10 percent because all I want to do is make a clean burning load and 10 percent gives me that. You may have to experiment with your loads to see what works best for you. I suggest you START at about 6 or 7 percent.
What I am saying here is that if you have been shooting a straight black powder load of 60 grains, you can reduce that to 54 grains of black powder and 6 grains of smokeless for a total load of 60 grains that you had been loading with straight black powder, but now your load is a duplex one containing both smokeless and black. It should go without saying that you must make sure your load has at least some minor compression so that the smokeless powder does not migrate. We are NOT trying to mix the powder. We are trying to get the fire started quickly with the hot smokeless so the black powder will burn clean and you don't have to mess with stopping and cleaning your rifle while you are shooting.
I have used Bullseye, Unique and 4227 for the smokless powder under the black powder. I like 4227 best because it seems to make the load burn cleaner, but that is a subjective judgement, and you may get different results. I do not use the 15 percent maxium with Bullseye. I use a 10 percent maximum with it and my loads are 7 percent when I use it, which is not often, and only if I run out of 4227. Unique works very well, but I limit it to a 10 percent maximum also.
If you intend to shoot NRA matches, duplex loads are NOT ALLOWED and they do check your loads randomly.
But for general shooting of black powder without the hassle of blow tubing and wiping every 10 or so rounds, using duplex loads let's me shoot as many rounds as I want to without doing anything other than loading and shooting. I've shot 60 rounds without puting a cleaning rod in the barrel and the barrel was about as clean after those 60 rounds as it was after the very first one fired. I believe I could have shot 60 more and the results would have been the same. You will be simply AMAZED at how much cleaner your bore is using smokeless under your main charge of black in any brand or granulation size.
A word of caution here: Using small amounts of smokeless powder in duplex loads is a technique designed to make your loads burn cleaner. That is all. It is NOT to make them more powerful, so don't try to make your rifle a black powder magnum PLEASE. We like you boys and we want to see you stay around here and talk shooting stuff with us and you can't do that if you blow your rifle up in your face.