Most of the scratches on my brass are from feed lips on new mags plus the sharp edges on the barrel extension. Some forum members polish or sand the feed lips. They also blend and polish the feed ramp and barrel extension lugs. After a couple hundred rounds my AR-15s wear in and the scratches go away or are not as noticeable.
If the 123 grain factory SST stick in the chamber after thoroughly cleaning the chamber and barrel it is possible your chamber is out of spec. A factory round should fall out of the chamber with the bolt open and the muzzle raised. On new barrels you might need to tap the upper to get the round to fall out of the chamber. If you need a cleaning rod to push the round out of the chamber good chance the barrel or ammo are out of spec.
If the 123 grain factory SST stick in the chamber after thoroughly cleaning the chamber and barrel it is possible your chamber is out of spec. A factory round should fall out of the chamber with the bolt open and the muzzle raised. On new barrels you might need to tap the upper to get the round to fall out of the chamber. If you need a cleaning rod to push the round out of the chamber good chance the barrel or ammo are out of spec.