Critter,
I have some experience with the Dead Center bullet line, including the duplex.
My current hunting load in my .50 cal encore is a 195 DC duplex over 100 grains of 777 fffg. It will easily shoot 1" at 100 yards.
I also have a knight wolverine lk 93 with the 22 inch bbl. In it, I am shooting the 220 grain DC over 80 grains of 777 fffg and it will litereally shoot a ragged hole group at 100 yards.
By the way, neither of those guns use a 209 primer as the ignition. My encore has a .25 ACP conversion and I use small rifle primers. The wolverine uses the #11 precussion caps.
I have never tried the duplex DC's in my wolverine as I stumbled upon the above combination early in the process and have stuck with it.
I did speak to a technician at Knight just last week, and he did make the comment about most Knights shooting better with less plastic between the bullet and the bore. In other words, a .44 or .45 cal bullet in a thinner sabot is likely to shoot better than the .40 DC inside a thicker sabot or a .357 DC inside two sabots. However, this has not been my experience. Also keep in mind that all the while he is telling me this, he is pushing the Knight Red Hot line of saboted bullets.
The sst/shockwave also seems to be a fine bullet, as my dad shoots them over 100 grns of 777 pellets in his encore and they work well. You may want to try those 200 sw's and 250 sst's over about 80 grns of 777. May be the ticket.
I personally have never been able to get powerbelts to shoot very good in mine or my dads guns, but there are others here who have had good luck with them. I understand that the Knight barrels are slightly over bore and the Power Belts are slightly under bore, which would seem to create a little too loose of a fit for great accuracy.
Good luck at let us know how your test session turns out.
Regards,
Smoky