I "HAD" a 317 No Dash that would get carboned up with rubbish after 4-5 cylinders full of ammo..
But after cleaning it was fine again..
I never used a brass brush on the cylinder being it was aluminum, only a plastic brush & Hoppes #9 on a Q-Tip..
I wonder how you are cleaning the cylinders bores??
Just a thought..
Good Luck getting it fixed or replaced again..