When I clean the breech plugs, I take them out of the inline rifle and to the kitchen sink. There I have a little dishwater made up (hot water works best). I have a nylon brush and I can soak that plug in that hot water and then scrub it spotless in a matter of moments.

After that I pour some isopropyl alcohol through it and then work it over good with Q-tips to make sure I have not missed anything. They look as good as brand new when I am done with them.