Welcome to Sig Talk, and Congrats on your MCX SBR kit!!
You've accomplished what many, including myself seems impossible. I mean, cobbling together the rifle - along with the necessary components from SIG, and the double dose of ATF paperwork - All while you're still walking this earth is an incretable achievement!!
Your copper flakes are a new issue for me. I'll suggest that these may be coming from a slight alignment problem with the barrel extension's M4 feed ramps. It's a guess that some copper could be shaved off the bullets as they're fed into the breech, and running suppressed has enough blowback pressure to push these flakes into the rear of the upper receiver.
Make sure the barrel's 2 positioning pins are fully inserted into their slots on each side. If the barrel's good, then call SIG customer service. They'll want to take a look see for ya...
Cheers