I would imagine IMR-3031 is still faster than BP. The other side of that is that BP is only 51% efficient. The 'leftovers' are the fouling in the bore and the smoke we see. Obviously that doesn't occur with the new fangled smokeless powders. So yeah, I can see where it would clean up a BP load but, I would never have thought to try it. The other thing I noticed in Daryl's post was that with IMR-3031 he was 4 grains over over the usual recommended 10% max. Given the slower burning rate of 3031 when compared to the powders we normally use for duplex loads, I can only assume the 'overcharge' doesn't present a problem.