Threads have to be cut on the barrel so it definitely leaves a mark. I had one onmy .338 Win Mag and I had to wear a combination of plugs and muffs at the range. One day I put the plugs in but forgot to slide the muffs on and I pulled the trigger. My ears rang for the next year plus. I hate those things.
I don't mean the threaded type , I'm talking about the ones you measure your barrel diameter then they make on to go over the outside of barrel and tighten down. They say they don't leave a mark but that's why I'm asking.