When you shoot a round of ammunition you're making something move at some 2300 fps in the barrel. That's a lot more force than most brushes or drills. Shoot the gun, then pour hot water down the barrel and on the piston, then clean the gun like normal.
If what you suspect is pitting is smaller after the whole ordeal, then it is not pitting and is some sort of buildup, and you need to repeat the process of cleaning and shooting again. It will progressively get better and you will shoot more. These are good things, cz858lover, these are good things.
You may want to look into the sponsor Canada Ammo, they are getting a new shipment of 7.62x39 that's non-corrosive in soon. A worthy investment, perhaps.