This stuff :
https://www.scalehobbyist.com/catag...uid-100ml/AMJ00002020/product.php?s=3&t=1&u=6
But gun bluing ( "cold blue " or " instant " ) is cheaper by volume , but you'll probably spend more than 10 bucks
They are both copper sulphate + selenium dioxide and an acid .
Well , I assume MiG's is that since they have identical appearance but you'll never find a friggin MSDS from them or AK on anything they make .
I just paint them with Rustoleum's modified alkyd rattle can and then add tonal variation with acrylics and pastels
The Rustoleum "' Painter's Touch 2X " in " Dark Walnut "
The " Espresso " is a good color too , but the dark walnut is darker and works better for track IMO .
Here's some other ideas .
I've never tried straight HCl acid , gonna need to see about that :
https://archive.armorama.com/forums/233609/