RE: Oil in barrel of ML' s, Is this a good Idea
To help eliminate confussion, oil in the barrel should be used when storing the firearm. When you intend on shooting that firearm you should remove the oil to eliminate excess pressure which can cause POI shift, be dangerous and damaging to the firearm.
If your worried about the elements effecting the inside of the barrel or getting your powder wet use electrical tape on the end, as this will prevent moisture from entering via the barrel. Safe and reccomended by many manufacturers and knowledgeable BP/CF rifleman.