Long paper on moly, worth readin the whole thing but I pasted applicable section below.
http://www.precisionshooting.com/aug98.html
I have heard that in a humid environment the sulfur in the moly will react with the water in the air to form sulfuric acid, and it will really eat up your barrel. Is this true?
A. No. Or at least, not under any conditions that you could live through.
Molybdenum disulfide is an extremely stable compound. However, any compound will decompose into its elemental components if you heat it to a high enough temperature. According to that technical bulletin, moly does not even melt before it undergoes thermal dissociation, which occurs at a temperature of 2910ºF, about 100 degrees above the melting point of steel! Now, that is what happens in a vacuum or an inert atmosphere. In the presence of oxygen, MoS2 oxidizes to MoO3 (Molybdenum trioxide), but only at temperatures above 300ºC (572ºF). Below this temperature the reaction proceeds so slowly, if at all, that the reaction rate is hard to measure.
At those elevated temperatures, it is true that the sulfur from the moly would react with the oxygen in the atmosphere to form sulfur oxides which would then react with water to form sulfurous and sulfuric acids. While this reaction can possibly take place, it does so only at high temperatures, and it is partially blocked by the surface formation of molybdenum trioxide and by any coatings of organic anti-oxidants on the moly such as oil, grease, or (carnauba) wax. At ordinary temperatures this just does not happen fast enough to be of concern. Maybe it might be a problem after a few million years.
Yes, the gas temperature inside your bore does go above 2000ºF—for less than one millisecond. However, for this purported chemical reaction to take place, there are two things that have to be present: water vapor and oxygen. There is some water vapor in powder gas formed by the burning of hydrogen that is present in the powder, but there is no free oxygen.
Even in the most highly nitrated versions of nitrocellulose, there is not enough oxygen for the powder to burn completely. There is always some carbon monoxide in the powder gases; and usually, there is some unburned carbon. Instead of the oxidizing atmosphere needed to cause sulfur to form sulfur oxides, the powder gas atmosphere is a reducing atmosphere that would tend to "suck" oxygen off any sulfur oxides rather than contributing to their formation.
Another rumor similar to this is that, at high temperatures, moly will decompose into molybdenum trioxide and elemental sulfur which are abrasives. The preceding paragraph should put the lie to that, simply because there is not any free oxygen hanging around from which molybdenum trioxide could be formed.
Even so, that technical bulletin is explicit in stating that molybdenum trioxide is not an abrasive! In fact, not only is MoO3 not an abrasive, the technical bulletin says it can be used as a high temperature lubricant for temperatures from 350ºC to 700ºC (that’s from 662ºF to 1324ºF). Note, this is below its melting point of 795ºC (1463ºF).
Elemental sulfur is not much of an abrasive either. It has a hardness of 2 on the Moh scale, the same as gypsum. This is softer than the minerals used in the abrasive bore cleaners!
So, don’t worry about this. This is just another one of those scurrilous rumors spread by ballistic Luddites who, for whatever reason, just don’t like moly-coated bullets. It seems that they don’t even like the idea of moly-coated bullets. This particular rumor makes no more sense than the rumor that moly in your bore will somehow suddenly grow into large crystals that are abrasive that will gouge great furrows in your bore. For a compound with the physical and chemical properties of moly, these things just do not happen.
Oh, just in case you are interested, that technical bulletin says that the lubrication qualities of moly are not affected by nuclear radiation. Moly will keep on doing its thing even if you and your rifle glow in the dark!
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All we wanted to know and then some.......Sam