That is gilding metal transfer from the bullets. As Jmoore said you can remove it with JB Bore Paste, follow the instructions. You can also use a chemical bore stripper like 7.62 or the foaming type. Make sure you follow the instructions. I like to use a product called Butch's Bore Shine. He was a great Benchrester and I got some of his product before he took it commercial. It now is part of the Lyman product line. I use it the way Butch did. Patch bore to get the big stuff out. Use a bore mop to soak bore and go to bed. Next day wipe bore clean with patch. ( Butch's is for non corrosive ammo )

In regards to corrosive ammo I would suggest you find a solvent made for corrosive ammo. Sometimes at gun shows you can still find gallon cans of the old orange nasty GI solvent made to clean corrosive ammo. In any case you want to remove the corrosive stuff first and then you can remove the metal transfer from the bore without such a rush. Remember that there can be corrosive salts trapped under the metal streaks.