My thoughts......
I'm looking at the pics and I think the rounds are being dented or ever so slightly bent while being fed from the ramps. This is likely because the mags are sitting too low (tolerance stacking perhaps) and the rounds are going in at an extreme angle as they feed in/up the ramps. With the bolt slamming at the rear of the cartridge at speed, I'm not surprised that this is happening.
I assume they chamber in nicely if you ride the charging handle all the way home, right? How about using an empty mag, and load one round in from the ejection port, and ride it home......
The "bent" cartridges are getting jammed into the chamber causing marks consistent with what you're getting.
....so I went to measure my CMMG lower at the front "cut" edge since it's a clearly defined line.... (8.47mm = .333")
...and took some feeding pics so you have something to compare against....
OK... off to clean my dirty rifle. :D