Quoted:
I can understand that but are you saying there is no coating that you could put on a phosphate barrel that would, a) realistically improve corrosion resistance, or b) is not cost efficient for the price.
You mention qpq nitride. Can this be applied to a chrome lined barrel and who does this?
You have several contradictions in your line of thinking. Are you looking for a coating to put on a phosphated (parkerized), chrome lined barrel, or are you looking at other alternatives? Most of the things you discuss are not compatible to just apply to the barrel you have.
As an example, as I already mentioned nitride is impossible with a chrome lined barrel. Nitride is a process in which the entire bare steel part is immersed in various liquid baths at high temperatures (~1000 degrees F) to achieve the chemical transformation of the surface. Metallic chrome plating would not only be destroyed but would ruin the chemical bath of the nitriding company in the process. Similarly a parkerized surface is not compatible with nitride and would have to be removed.
Ion bond is similar, it would need to be applied to a compatible surface, which parkerizing (AFAIK) is not.
So you really need to clarify your question. Do you want to add something to the barrel you have, or are you starting over with a new barrel for your ultimate corrosion resistant finish? My first response in the thread was my most reasonable suggestion for something effective to coat your existing barrel with (moly resin), in combination with the porous nature of the parkerizing this would create a very durable and corrosion resistant finish.