Charcoal! I've been just about as miffed, in my case, I attacked the bore with tight patches and valve compound....I was NOT gentle, but it was like a new, good barrel after and I got four seasons of good shooting out of it....
The tang M77s are kind of tricky and need a lot of careful work to get a "click" bedding around everything, even the middle screw, which really, really needs a pillar or sleeve. Mine ended up with three pillars (two sleeves in the back) and bedding around the front of the action and an inch of barrel. The action screws all go to a hard stop with no springing. But it was work to get there.
