My otis ripcord got stuck in the chamber of my MK18 when I was cleaning it. The barrel is an FN 10.3". I had to place the MK18 on a bev block on my vise and use both hands to pull the cord through. It was extremely tight and I felt/heard the brass end scraping against the bore. Looking through the bore with my eye it doesn't look like there's a scratch or anything, but the brass end of the ripcord has a nice scratch on it. I'm sure it didn't do anything since bullets are travelling through the bore at much higher speed and pressure. There's no way the chrome lined bore is scratched right?

Brass is softer than steel or chrome for that matter.  Don't sweat it.

Read up on Mohs hardness scale.

Lol. Your rifle bore scratched the brass. Not the other way around

The brass is too soft to scratch the bore. However, there could be harder grit embedded in the brass that could scratch the bore. Wipe any brass (or aluminum, or plastic, or anything really) clean before putting it down your bore.

My very first AR build lasted from late 2013 to about the summer of 2018. The worst of it was in the 17-18 period where I bought about 7-8k worth of Tulammo for it and shot 1k round per month until it finally died.

Anyway during that time, I did a lot of things including getting an otis patch stuck in the barrel and having lead build up from the .22lr conversion kit (that I still use today!). In both cases, I used a steel sectional rod, the USGI one, to hammer it out. Yes a steel rod, and a pretty heavy hammer. Once the barrel died, I looked through it and the only damage was due to the excessive firing. It completely swallowed a throat gauge.

I learned a lot more now especially how to clean and what sort of chemicals and how the brushes should look. None of my rifle barrels, except one, looks as bad as the first build. The other rifle that has a really bad bore overlapped the timeframe that I was going through my first build.

So yeah, consider it your learning platform. Chances are your barrel is okay but personally I think you should abuse the heck out of it.

If there is sand or dirt in the cord, yes, you can scratch the bore.

Like another poster said, the barrel scratched the brass.

I've seen more of those Otis Ripcords break than the Bore Snakes version- about 6:1 ratio. I had a Ripcord break in a Model 7 7mm-08 and had the devil's own time getting it out. Ended up yanking the barrel to get the piece out of the bore. Never again Otis.

I had an Otis ripcord get stuck in my Geissele URGI upper. The outer layers got piled up and it wedged in the barrel. I took the threaded end of a cleaning rod and got it started on the female end of the ripcord (which was sitting around the throat of the chamber) and was able to pull it back out. I'll never use a ripcord on a rifle again.

Think about what the copper bullet is doing as it screams down the bore at 3000+ fps with 50k psi behind it. Then stop worrying about it.