Those appear to me to be from the inside(back) edges of the barrel extension. There is a good ridge of displaced brass towards the case mouth under magnification at those spots. If it was rotating the brass would have gone more radially. Im guessing it whacks those lugs, bounces off a little and then gets dragged the rest of the way out making contact w/ only the two points as it sits in their valley. I stoned the back edges of the right side lugs last night so we'll see if the marks change. They were very sharp.
This looks like its going to be a fun little upper. Have a Colt mag/adapter already working and will try out Endomags soon My new handguard won't get here until next week though so it'll still be running naked this Sat. My old blowback setup was finicky w/ ammo and gassy when suppressed. I haven't used it in a long while. Really looking forward to playing w/ 9mm again.
Game changer from direct blowback.