Two thoughts
1. H-110 does not burn well at low pressures, so the "residue" that is sometimes hitting you in the face might be unburned powder from the occasional load that stops burning too soon. Your 15.5 grain load is about 7% below max, and there have been warnings to not reduce H-110/WW-296 (same powder) by more than 3% below max. But, on the other hand, when SAAMI changed the .357 Magnum pressure standard from 46,000 CUP to 35,000 psi, some manual revisions showed the new max charge weight to be more than 3% below the old max. So, it is hard to figure.
2. You made a point about the residue NOT being copper. So, I am guessing that you are familiar with the issues involved with the cylinder not being aligned properly when the round is fired. Still, it might pay to take note of which chambers of the cylinder are giving you the residue. One or more of your chambers might be larger than the others, resulting in slightly lower pressure when the cartridge in it was fired. And, don't rule-out alignment issues until you have carefully checked. I had an experience with a Security Six where the cylinder was not being locked when I was mid-way through a cylinder-full of loads. At that point, the weight of the unfired bullets was all on the side that needed to go up as the gun was cocked, and the imbalance, along with a too-thin cylinder pawl, was failing to get the cylinder notch to the locking "bolt" when I SLOWLY pulled the trigger in double action to test ammo. So, alignment was off and part of the jacket was shaved. So, I suggest that you pay attention to which round in a series spits residue, also.
SL1
