Store bought patches (especially prelubed) and Bore Butter do that...

Go to WalMart and buy some cotton pillow ticking...Mix up 2 parts bees wax, 8 parts Castor Oil and 1 part Murphy's oil Soap...This is the best lube I have used in about 30 years of messing with these things...

There was an article a few months back in Muzzle Blasts showing Bore Buttered patches, looked about like yours...I've tried it as well in my .54, without a wad between powder and ball, mine looked like yours as well...

Crisco and SnoSeal work pretty good as well...