In general, if it's a gun barrel, then Steve's method is good. If it's a barrel sleeve, such as the perforated sleeve on a .50 cal or a MG17, then these were black anodised, which after time often tended to take on a slightly brown tinge. For these in most model scales, then a very dark grey, almost black, is adequate. The perforation holes can be touched-in with black, for emphasis, although in small scales this can be tricky!