Very. Wish could offer more, but without seeing it, at this point it’s just speculation. A head scratcher indeed. You need to pull the barrel so you get to the lugs. I actually have a good bit of Silicone Nitride media I’ve used on bolt & lug mating surfaces. Mix the media with grease and work the bolt in the Action. Like this: https://www.youtube.com/watch?v=uyQMNY6fwxw

I’d be happy to send you some if you think it might be in that.