Dry Moly is somewhat hygroscopic but that isn't usually much of an issue in gun barrels due to a variety of factors (other byproducts of fouling). Usually, the issue people seem to have is the build up of the moly......too much of a good thing and a sludge they don't seem to understand. Moreover, the usual cleaning products have issues removing the sludge of moly+powder+lead+copper fouling.
In short, people misunderstand the use and application of Moly and how it actually works. The moly coated bullets are an example of too mcuh of a good thing.
Personally, I clean a gun real good, then in the oil the barrel step, I apply a light coating of moly infused oil. Heat and friction will make the moly bond to the barrel. You do not need a lot of the stuff to do the job. I have most of a 1 lb. can of dry moly that is about 20 years old...more than enough for my battery of guns, projects, and other uses.
just my $.02