If I plan not to shoot the gun for a long while, I will clean it out.

But if I am actively working up a load or working on a rifle, or its hunting season, I will just clean out the powder and oil it.

Copper oxidizes faster than steel and leaving it in for years and years, can cause pitting.

I get hot on working on a gun and then put it away and might not pick it up for a year or more. Or forget.