I'll first explain the reasons for using duplex loads.
1 . In the 45 ACP loads described it was to get a slow recoil surge, obtained by a long low pressure push rather than the normal sharp rap produced by single powder charges, like your A#7, which leaves some unburned powder. My duplex loads did too, but produced exceptional power at very low pressure.
2. Duplex loads are always used to get a slow burning or hard to ignite powder burning, or make it burn clean so will most often be used in rifles with considerable case capacity. They will probably be found most useful in making surplus cannon powder work in cartridges where they don't develop enough pressure/velocity to make them usable.
Follow this one safety rule and you won't get in trouble. ALWAYS COMPRESS THE POWDER. ALWAYS. ALWAYS!!
The mechanics are as follows.
If a full case of powder is too slow to develop desired pressure, reduce the main slow powder charge and add a faster burning powder under it, so the total load is compressed. Do this gradually and you'll need to test fire each load immediately. If you have to go to a range to shoot, forget it. You will be open for confusion and trouble. For an example: I bought a bunch of very slow burning cannon powder which would only push 30-06 bullets to about 1800 fps with a lot of unburned powder. Lets say a compressed load was 60 gr. I reduced it by 2 grains, and dropped in 2 grains of Unique over the primer. Better speed and pressure, less unburned powder, but still very low velocity. Repeat, but put in 4 gr Unique. The fast powder can be anything relitively fast, for 30-06 it could be from 700X up to 4198. The object is to get enough fast powder in to raise cartridge pressure up to a good burning rate for the slow powder, after which it burns along smoothly and clean. So, for illustration purposes, perhaps it required a final load of 10 gr Unique and 50 of the slow powder to get good working pressure and velocity. It would have taken perhaps half a case of 4198 to do the same thing, or perhaps 6 gr of Bullseye, 231, 700 X etc very fast powder. Exactly the same thing as using kindling to start large wood burning in a wood stove, only it happens quicker. In the case of the above 30-06, slow surplus powder loads, I got full power and clean burning loads using duplex, using a powder that I had no other use for, and it was EXTREMELY inexpensive! I didn't try to exceed what I could get with single powder loads. (Note that slow and fast powders very greatly in density. This means the fast powder charge increase will be different than the slow powder reduction, as I stated in my illustration above. When you reduce the big charge, put in enough fast powder to get compression.0
With proper powder selection it is possible to develop loads which will exceed the power of any single powder charge, as I did with the 45 ACP, in any cartridge. But probably the average well experienced re-loader will find duplex loads most useful sometime in the future when powder selection may be from large military shells found laying around, etc.
Between you and me and the gatepost, ol Veral has never been to a shooting range, ever. Drove past a few, but never stopped in or shot at one. Most of my life, I've lived in places where I can load a cartridge, step out the door and pop the primer. That makes a lot of playing easy to do, and development of duplex loads or whatever, quick and easy. If I hadn't been able to load and shoot this way, LBT would have never gotten off the ground, and I wouldn't be writing this forum.
If any of you have more questions regarding duplex loads,, throw them in and I'll answer. The above is quite brief for the subject, but I can't offhand think of any more answers till I know what your questions are.
