On a hunting trip, I've used Rig for coating the underside of barrels, in the channels, etc. The stored ML or modern gun on a hunting trip has rig everywhere - patched out of the bore then solvent cleaned and dried before shooting - ML barrels for me, last just fine in all conditions around here, with nothing but the WD40 that was used as the final cleaning, oiling - has for many years.
We - Taylor adn I, never get any rust in the bores of our guns.
If rust appears after cleaning, drying and WD40'ing, then perhaps you didn't get the barrel wasn't as clean as you thought. I am wracking my brain trying to understand why someone would get rust, when we don't- not even a smidgen or a whisper or thought of rust. This is the only thing I can think of - I'm just throwing this out there as a possibility.
Perhaps the barrel should have been removed from the stock and cleaned by flushing water in and out until it was clean, then dried, then flushed with WD40, not just a damp WD 40 patch, but flushed with the stuff,THEN that is patched out to remove the excess, blasting WD40 10 to 14" out the vent with that first, second and perhaps only 4" with the thrid stroke of that patch - sometimes, not always, changeto a fresh patch for a final bore wipe, then wiped down with the wet one, then stored muzzle down. THAT is pretty much how we do it and we get no rust - ever.
1 day or 10 days or 6 months later, a dry patch down the bore comes out either dry (if the carrier has evapourated) and clean, or wet and clean - no colour at all - absolutley no rust.
Lately the humidity has been in the 80% range - over the year, it averges about 50% - I'vestored ML guns for years - no rust - they were CLEAN before being stored, WD40's then stored. They had been cleaned by emersion of the breech in a bucket of tap COLD water, water flushed in and out with actual hand and arm pressure on the rod - deliberate cleaning, I like to call it, not merely humoured, ie: patched out with the barrel on the stock. Cleaning the barrel with it still on the stock does not get them perfectly clean - imho- at least, not clean enough for me. I use only WD40- I don't get rust.
I didn't get rust when I lived on the coast in the Vancouver BC area and I don't get rust living inland. If the gun rusts, it is being wet somehow, or wasn't perfectly clean and that residual fouling is absorbing moisture somehow and causing rust.
With the number of guys here who don't take the barrels off their stocks for cleaning being very high, perhaps that is why a super rust preventative is needed and normal oils that work elsewhere, just don't work.
By all means use whatever is necessary - but I sincerely hope I've made some suggestions that help someone who is having trouble.
Method is everything.