I’m not sure how it happened but my AR15’s upper reciver has rust and pitting inside the barrel in the boor.  I’ve used several different types of brushes but I can’t get the bore smooth with just regular cleaning.  So I need some subjections on what I can do to fix the barrel.  

If the barrel is truly pitted, you will NEVER get it smooth again.

Pitting is like a chuck hole in the road, but you can repave or otherwise line the bore with anything that will stay in.

The pitting will fill with jacket material/fouling more than likely. It will be a bitch to keep clean, if you really can. Accuracy may/will be affected.

It takes a lot to pit a barrel, how did you store it? Did you clean enough and properly? Do you know what pitted it and how? I would want to figure it out so as to not have it happen again.

There was a bumpy texture on the inside bore and there was rust in it.  It took a long time of working the barrel with a brush to get the rust out and to make it shiny again, but there is a bumpy texture to the rifling now, very smellier to very course grade bead blasting, like with something the size of bebes or something.  

The upper was stored for a long time in a dresser and there was only minimal surface rust on the outside of the barrel, just a few spots on the entire outside of the barrel.  I think after it was last used it wasn’t properly cleaned and the ammo might have been corrosive.  That’s the best I could come up with.  Everything thing else, the bolt and all the other metal parts were ok.

The barrel is a RRA chromed line barrel, so I never thought it would rust like this.