New BCA upper...First time shooting today. Everything seems to function fine. No charging handle to the teeth, bolt stays back after last shot, fed and ejected each of the 5 rounds I shot. Only shot it 5 times since it was mainly to check function. Plus I had the upper mounted on a pistol lower with the blade arm brace.
Anyways, after picking up my spent casings, I looked over them. Upon inspection I noticed about 1/3 to 1/2 way around the rear groove by the rear end, that there seems to be black sooty type stuff. I also noticed a few scratches on the brass going from back towards front. I did use lots of slip 2000 oil on the BCG before firing the first shot. Could the soot film be burnt oil? Blowback from gasses for the function? Anything I should be Worry about? Thanks!!
****Edit****
After looking a I can see a little of the residue down the side of the case leading to the ring. A few don’t seem to hv as much, so maybe it was clearing up?
Full pics available in these links
https://i.imgur.com/K5XcN3v.jpg
https://i.imgur.com/nh5lPm8.jpg