Find some brass ''screen'' material. You may have to order it online.
Wrap it around a worn bore brush, until it fits snugly in the bore and run it through a few times.
When I used this method, I soak the bore in liquid detergent overnight by plugging one end and filling the bore.
This really softens up the ''crud'' you're talking about and a few passes with this ''screen brush'' will get rid of most of it, to the point it's easily removable with regular cleaning tools.
You say the ''crud'' is rust.
Are you sure it's rust?
It might be lead fouling that has rusted underneath over time.
Some nations used to issue ''metal screen patches'' to their troops to clean the rust and other bits out of the rifle bores when they went a long time between regular cleaning.