Homer, what powder and charge are you using in your 1600 fps loads for your 404?
I load cast to much that same level in my 404 just for cheap easy shooting and use Vectan AS powder. This powder is equivalent to Red Dot in burn rate but is so incredibly clean burning leaving only a stain in the barrel rather than any soot or little bits of unburnt powder. I load 22.0 grains with a 360 - 400gr cast in the 404, the exact same powder and charge I have used for decades loading 32 gram shotshells.

Vectan advertise their AS powder as suitable for reduced rifle loads.