Hoppe's No. 9 is a nitro-powder solvent, NOT A BLACK POWDER SOLVENT!! I have tested it thouroughly for removing BP or other BP-substitute powder fouling from steel, and it DOES NOT WORK satisfactorily! It will remove some, albeit slowly, and leave some too.

You must make sure you get ALLfouling out of your bore, or rust will eat it up in short order.

I have been cleaning mine since 1964 by pouring two quarts of boiling water through the bore, letting it drain out the nipple seat hole (of course, you have already removed the nipple and put it in a glass of hot, soapy water to clean it after you get the bore clean!). If you have an inline, pour the water through from the breech, using a funnel of the appropriate size. After the water has drained out of the bore, swab the bore with four or five clean, dry patches-the heat will dry the boreimmediately, and it is now clean. Once the barrel has cooled, swab it out again with a patch saturated in Birchwood-Casey Sheath. This absolutely prevents a clean barrel from rusting. And it dries out, so you don't have to worry about if you got all the oil out of the gun before you load it up again!Swab all powder fouling off the outside of the gun with a damp patch, and after drying the metal parts, swab them with the patch you used to apply the SHEATH to the bore. This stuff won't hurt wood finish, and keeps all ferrous metals from rusting.

Now, if you insist on using some kind of solvent other than water (water is the universal solvent in this particular universe!!), I have found that the SHOOTER'S CHOICE gel works very well. There are, no doubt, other BP solvents that will work also, but it is my understanding that BM3, like several other substitutes such as Clean Shot, GOEX ClearShot, Pinnacle, T7, APP, etc., are best cleaned out of your bore with just plain water. Try water, and forget the Hoppe's No.9!! After you get ALL the fouling out, use Birchwood-Casey Sheath to preserve the bore. (Yes, that "brown stuff" is as you suspected-RUST!)

I have never had to use anything as harsh as a bore brush in my ML rifles.