Ask 100 BP shooters what to use to clean a muzzloader, and you'll get 100 different answers. (Well almost)
On half stock rifles I take the barrel off and pump with hot soapy water, dry, one patch of WD/40, then dry patch, and use a good gun oil, or bore butter. Like GB.......No problem with flash rust, unless you wait too long to dry, and that ain't no big deal! It comes off easy.