ive seen so many people see the black on the bullet and think AP right away. Magnet test is really the only test to see if it has a steel core other than shooting at a piece of steel. I buy range bullets, so many of the 30 cal are steel core, wash them up and reuse them because nothing is deformed on them.
So many people can get a marker and make black marks on ammo. i saw a vender at gun show once, with a sharpie "making" AP ammo, i just laughed, but someone else saw it and told the show's promoter and they escorted him out with his stuff,