Copper filings on brake

If you don't have any dings in the brake, indicating an off-center brake / bore axis, then it is just fouling that got blown out of the barrel during firing, and collected on the brake baffles. Perfectly normal.

It's normal.
People don't think that copper plated bullets can copper up a rifle, but they're often astounded at the amount of copper fouling they get out of an AK-74 muzzle brake, and out of the gas block.

I found by accident that Shooter's Choice Extra Strength Copper Remover also does a great job of removing the black carbon from the muzzle brake and gas block.
I could never get the baked-on carbon out of my Bulgarian hard chrome lined brake no matter what.
I tried soaking it in Hoppe's, and with Slip 2000 Carbon Killer and it and the gas block were still black.
A short soak with the Shooter's Choice Copper Remover and they were down to the hard chrome lining.

The Breakfree bore foam does a very good job of removing copper and carbon. Eats both off and leaves the hardchrome bright and shiny.

BSW