This is a new gun. I'm sure those mags are new, too. They need broken in just like the gun does. Load all your mags to the top, FULL, and let them sit for a few days.
Cycling live ammo into the gun, and manually extracting it - that's what's leaving the brass flecks. Bet your brass is marked up, too, with lines in it. That's the ejector, pushing the brass to the side, and the brass getting eaten up by the inside edges of the barrel extension. That'll happen alot less during live fire, and then happen even less once the gun is broken in.