Jestertoo: What’s worked best for me is to pull the bullet and dump the powder which is almost certainly the cause of the verdigris. Then soak the case and bullet for a few days in cutting oil or something similar. Then you can clean up the metal with some bronze wool that won’t alter the patina if you don’t get too aggressive with it. If you’re concerned about proper weight, you can replace the powder with an equivalent weight of corn meal and then reseat the bullet.
It might also come off with brass wool without altering the patina of the case if you’re not able to pull such a large bullet as described above (if that is a .50BMG?). Falcon here on the forum had made some giant large-caliber inertia pullers a few years back which is handy for pulling those.
It’s 12.7x120 .50 HV so pulling the bullet isn’t easy. I’ll see what I can do with brass wool and wax.