You see the term used a lot on old Enfields.
The British Commonwealth Countries used a propellant
called CORDITE in their MK VII (Standard Ball Loading)
.303 ammunition, The propellant looked like Spaghetti noodles
and burned extremely hot. If not properly cleaned afterwards, they left a strange color of rust in the bore that appeared to be a silver gray
color. (Frost) After cleaning the bore would be dark in some places
like it was burnt. (I have several Enfields that have the dark bores)
This propellant would also cause a great deal
of bore erosion in a very short period of time. It is a rare occourance
for a person to find a #3 Enfield with a REALLY good bore because of that ammo.