Just finished the build below and took it out to the range for the first time. Gun functioned and cycled flawlessly. Groups were initially horrendous (7ish MOA at 100), but closed down to about a MOA after 60 rounds....this in and of itself is nuts to me. I've never seen a gun do that. I only mention it in case it's relevant to below...

With that said, I opened it up to clean her tonight after about 60 rounds of Hornady 208 Subsonics and I'm seeing this yellow grit. I'm thinking it's unburnt powder, but I've never seen this much before. I've also never really shot subsonics supressed out of an SBR before. My worry was also that it could be brass shavings and something's wrong with the build...but I saved all the brass and it looks pretty normal.

Thoughts? Tips? Advice?

Thanks in advance

300 Blackout
8.5" Noveske barrel and gas block
Black Rain Ordinance Nickel Boron BCG
Larue Stealth Upper receiver
Seekins SP223 Lower
KAC URC Rail
Surefire SOCOM Can and Brake
Larue Grip
Larue RAT Stock
JP Silent Buffer
Nightforce SHV 3-10
GG&G QR Mount

Yes, it's powder.  Somewhat common with some brands of commercial loadings when used with a suppressor.

H110/W296 ball powder is difficult to ignite, especially in subsonic rounds. I was getting this until I switched to magnum primers in my super sonic loads. I don't use ball powder for subs because of residue. IMR 4227 works good for subsonic loads.

Possible filler for subsonics?

Interesting thought.

I do reload a lot...but I've yet to see powder turn gold...

It's only gold in the store when you buy it!

Its filler. Same thing in the gemtech subsonic rounds. Pull a bullet on one and look. Thats if you have a bullet puller.