3 probable places it's happening:
1) Loading gate. I think this is most probable because of the waviness of the scratch. Also, the gate is a stamped part and a lesser version of this is quite common. To check, load a cartridge, push in the loading gate and eject it through the loading port. You'll likely have two scratches and it will probably catch the bullet too.
To fix this, deburr the loading gate cover with a hand file. It's really hard metal so it will take awhile. Don't get it hot. If you take the bolt out per standard cleaning instructions, you can remove the loading gate through the bolt opening - otherwise, follow the disassembly instructions on maurader's website.
If that isn't it, you're taking the gun apart. Follow maurader's instructions being careful to pull the trigger while you remove/install the hammer to protect the sear.
2) It could be a burr on the base of the carrier. To check, remove the carrier and examine with a finger. Deburr with a file.
3) It could be a burr in the chamber. To check this, load and unload a round through the ejection port a few times and check for a burr. Nothing is coming into contact with the case wall except the chamber during this process. If you get a burr here, see a gunsmith unless it's an obvious simple fix.