I keep one of my C&B revolvers loaded always, just as I do my more modernistical pistols. And have, in the past had a flintlock stay loaded for two years - worked fine when I "unloaded" it at the range.
I do keep the frizzen covered with a leather shroud, and a toothpick in the touch hole.
In certain un-named Asian countries, a condom over the muzzle was used to keep the rain out. I imagine an M-60 with a barrel full of rainwater would not react well were the trigger pulled. Have not tried that with an ML.