Tuzo,
A cautionary tale for those unhappy about rust.
What you describe is also sounds like exactly what you see when the powder has begun to deteriorate, whether water is involved or not. I had some .308 surplus ammo from the former Eastern Bloc that had some good, some weak, some failure to fire. Pulled it down. The stick powder in them was dry in some, clumped and sort wet or oily looking and had to be dug out with a probe. In a few instances, I identified verdigris on necks over holes that had corroded through. That was due to nitric acid radicals evolved off the powder as gas. Indeed, one side effect of shooting the stuff in my M1A was that about a month later I looked at the bore and discovered a coating of fine red rust, same as nitric acid fumes induce on steel being rust blued by the old steam cabinet method.
Anyway, if you take some of the clumpy powder and put it into some water in a shot glass and if you get a reddish-brown tint to the water, you have breakdown occurring. If you don't, then it's probably just wet, though if you happen to have a pH meter to test the water, that would tell you better. Just put both some of the dry powder and some of the clumping powder into water to see the difference. The pH gets pretty low even with small quantities of nitric acid.
The point of all this is, I want to advise anyone who reads this and has a similar sounding experience shooting ammunition like this to treat the bore of their rifle as they would a gun fired with corrosive primers. Use a bore cleaner that is water-based and has a corrosion inhibitor in it. Bore Tech's Eliminator or their C4 carbon remover are two I would suggest if you think the bore may have been exposed to powder in this kind of conditions.
Back on the topic of rolling water-proof loads: though ancient technology, the bullet sealant used by the military for decades was asphaltum (pitch) dissolved in mineral spirits. You can buy this today as
liquid asphaltum from art and printing supply stores as an etch resist for engraving plates. The military cartridges use this as a sort of bullet glue, painting it inside case necks and letting it dry before seating the bullet. If you've pulled military ammo down, you've seen it. The case necks were sized large (bullet slip fit) to make room for it. If you pull down military match ammunition (no crimp) and use mineral spirits to dissolve the asphaltum off the bullet and the inside of the case neck, you find you can push the bullet in with your fingers.