You might try one of these (with care):
M-Pro 7
This is good stuff.
Also:
SLIP2000 Carbon Killer
Not quite the miracle product it's advertised as, but it will take it off. There is a caution about leaving it on blued finishes for too long, so DON'T.
Like the sage ol' Smith smith said to me once, "Shootin' guns is a dirty business."
ETA links fixed
M-Pro 7
This is good stuff.
Also:
SLIP2000 Carbon Killer
Not quite the miracle product it's advertised as, but it will take it off. There is a caution about leaving it on blued finishes for too long, so DON'T.
Like the sage ol' Smith smith said to me once, "Shootin' guns is a dirty business."
ETA links fixed