In response to a recent query about coatings for the stainless steel M1 Garand gas cylinder, gas cylinder lock and gas cylinder lock screw, a brief explanation:
As manufactured, the stainless steel gas cylinders were (literally) "in the white"; a variety of coatings was tested, including Armor-Vit and Silco Black. Eventually "a molten dichromate black" was found to be most durable. Winchester gas cylinders have been observed (that) were iron plated and blued. In adition, Winchester gas cylinders have been observed which appear to have been copper vapor plated and blackened" (Quote from Duff, "The M1 Garand: WWII," page 147).
Today vendors such as Brownells offer various paints and other finishes, some of which are baked on. Maybe they work, maybe they don't. I'm just an old-fashioned guy who figures that Springfield Armory solved the problem in WWII, so why not follow their lead?
On the other hand, field repair/depot repair differs markedly from mil spec manufacture.
The process necessary to get the real blackening process requires a very complex system. So, other protocols are necessary once a component leaves the "factory."
Example: The M16 upper & lower receivers (aluminum) require a hard coat anodized finish. Once in the field, such a coating is unavailable. Thus, "spray can" coatings are in all the manuals. Nonetheless, hard coat anodize is the mil spec coating.
--Walt Kuleck & Clint McKee
See the next two links for the TM describing the original factory finish (Caution: Very Large Files!):
Gas Cylinder Finishing Page 1
Gas Cylinder Finishing Page 2