You don't need to scrub, about 10 passes with a brush and solvent is enough.
You need to let a copper solvent have time to work.
After brushing run two soaked patches down the bore and let soak for the recommended time on the bottle label, about 30 minutes.
Some solvents, like Sweet's 7.62 can be soaked no more than about 30 minutes, some like Hoppe's #9 can be left in for months.
After soaking wet a clean patch with solvent and run it down the bore and out the end in one smooth pass. Don't "pump" the patch up and down.
When the patch comes out, inspect it for green or blue copper stains.
If you see any, soak longer.
Another method for a badly fouled bore is to buy a jar of JB Bore Paste and use it to clean the bore.