JonM, He's shooting a Mini-14 not an AR, no bolt carrier and no buffer.sign of a couple possible things. overgassed (possibly), underweight buffer (very likely), light weight bolt carrier (not liekly), understrength recoil spring (possible).
whats happening is the bcg is cycling so fast the brass is thunking the rear of the ejectio port or brass deflector really hard. the scratches are a sign of that as well as the brass is being ripped from the chamber before pressure has dropped enough.
you can correct overgassing by heavier buffers and fullauto bolt carrier groups.
Your problem is too much gas pressure, causing marring on the cartridge rim, denting the case midway on ejection and sending the cases into the next county. The problem is a common one with the Mini-14. The problem can be solved with using reduced sized gas port bushings or an adjustable gas block.