I believe the length of time you can get away with leaving a BP rifle loaded without damage to the gun, depends a lot on the lubricant used on the patch.
In Vladymere's instance off the chambers being pitted, I'm not sure.
BP powder as it is, is usually not particularly susceptible to absorbing moisture on it's own. Perhaps it is possible the Ruger was not well cleaned before loading, or the powder used was GOEX or other brand from a period when the powder actually broke down due to the potassium nitrate used in the mix.
Black powder substitutes that use perchlorate in the mix should not be used in any ML gun. That is all I will say on that subject, other than perchlorates ie: chlorates fouling is VERY corrosive to iron molecules. BP fouling causes oxidation of the metal surface it is on, when the humidity is above about 35%.
Chlorate and perchlorate fouling does not need the humidity to be that high (35%) in order to absorb moisture from the atmosphere & turn into an acid which actually dissolves, has an affinity towards iron molecules.
Use perchlorate 'powders' at your own peril.
When using track's mink oil or Neetsfoot Oil, I have left my 14 bore that was "fresh" loaded for up to 3 months, without damage. Mind you, it was not fired, then reloaded & left loaded.
"Back in the day", guns were poorly looked after, were shot very little compared to what we do today with them. As well, even though they were not shot much, their bores needed re-boring and the rifling "freshed" often. This was due to the quick wearing of iron & the poor care of the guns leading to their bore rotting state.

EDELWEISS - you have probably heard or are well aware of the damage caused to a bore by using "corrosive" primers. Those contain just a tiny smidgen of the chlorates used in many of the phony powders, which can total up to 17% of their composition. Think about that quantity for a minute.