I have shot straight smokeless loads under PP bullets at 2700fps
and straight BP loads with my DDEPP bullets to good effect.
I see no reason why the DDEPP bullet should not work with duplexing. 
beltfed/arnie

Arnie,  Do you find it necessary to wipe the bore when shooting duplex loads with paper patched DDPP elliptical bullets?  To be specific, the load I am trying is 6.5 grains of Reloader 7 and 70.5 grains of Swiss 1 1/2 with a BACO Jim 453546E bullet cast with your formula of 17:1 lead and lead free solder.

My ladder test using Swiss 1 1/2 with this bullet showed a sweet spot between 76.5 and 78.0 grains of black powder so I am starting at 77.0 grains.  I want to develop a duplex load for gong matches where most people are shooting smokeless and the maximum amount of smokeless recommended for duplex loads is 10%.  I wanted a little less than 10% smokeless and 6.5 grains is 8.4% of 77grains.  So...maybe not a good reason or not wise but I have to start somewhere.

OK. Thanks for replying. I wondered, with such finely identified numbers if you'd already tried this out. Well, you can see where the mystery began....
Still don't follow your math but, I've done some peculiar stuff myself. Carry on, and let us know how it goes.

I think the real test of whether the duplex loads will be useful will be the shot-to-shot repeatability of velocity.

CHRIS

Back to Pilgrim's original question, "Will paper patched bullets shoot with about the same, better, or less accuracy when using a duplex load vs a  straight black powder load"?

I'm curious as to the answer which was not provided in the previous comments.  I just sold a Browning .45-70 BPCR to a guy that plans to duplex paper-patch loads.  I was under the impression that fouling was reduced but have no idea as to accuracy results vs straight black.

My best guess is it would potentially decrease accuracy a bit due to the leading edge of the pressure curve.  Shooting straight BP results in a very fast leading edge (rise time) of the pressure curve vs smokeless.  The fast pressure increase quickly obturates the soft lead PP bullet before it starts to move into the throat resulting in a good gas seal & good alignment with the bore.  Due to the slower rise time of the front edge of the pressure curve, smokeless will tend to push the bullet into the throat before it fully obturates, allowing for some possible misalignment.  Just my thoughts & I could be all wet.

Wayne

Piltdownman,

Black powder is very efficient in these guns.  So much so that I use it almost exclusively and am only looking into duplex loads to use in gong matches where you can get pushed for time when wiping or blow tubing.  Many of the shooters in gong matches use smokeless powder and the pace of shooting is fast.  In my ladder test I loaded 16 rounds with a 1/2 grain increment of Swiss 1 1/2 black powder between each round.  I started loading with 73.0 grains and ended with 80.5 grains.  All of these rounds were shot at one large poster board with only one target dot on it.  The total vertical distance between the lowest and highest shot impact at 200 yards was only 2.98 inches.  Interestingly enough, the vertical impacts did not linearly track with the powder charge (76.0 gn was the lowest and 78.5 gn was the highest).  As you can see black powder is efficient enough that there is very little difference between impacts but with this test you will usually find a range of powder charges where the vertical spread is very small in this case only 0.222 inches of vertical spread between loads with 76.5 to 78.0 grains of powder.  Somewhere in this range you should find your best load.