What worked for me:
Step 1: Soak several cleaning patches in Montana X-Treme Copper Killer and stuff them down the bore until full. Put in a ziploc bag and allow to sit overnight. Fouling should scrub out easily after that. Any stubborn spots can be removed with JB Bore Cleaning Compound on a patch wrapped around a jag and applied vigorously.
Step 2: Go back to using RMR\’s excellent copper-jacketed bullets.
I love RMR\’s products in general, and I like the fact that they think outside the box, but the zinc-jacketed bullets simply don\’t work for me.
I echo all of the OP\’s caveats regarding advice from other users. I would add that I know how to load Hi-Tek coated bullets so that there is no swaging of the base or damage from over-crimping, and if that careful process isn\’t good enough for a particular bullet, then that bullet is not for me.