I am a full-time gunsmith and started my career doing restorations on antiques back in the early 70s. Because of the job, working on old guns was the norm.

I bet I have working on at least 2 dozen guns that were loaded when they came in and all those that had not been wet were fully capable of firing.
Black powder has 3 basic components. Potassium Nitrate, sulfur and charcoal. None of those will break down with age. So if the potassium nitrate is not devolved and washed away it never goes bad, or breaks down. Sulfur is a natural mineral that is found and mined in the fist place. Charcoal can break down over hundreds of years by sunlight, but if kept dark it will last as least as long as the pyramids in Egypt (it's been found in sepulchers and was as good when found as when it was placed there.)

So if the gun is kept correctly and not immersed under water, a weapon loaded with black powder will fire no mater now long ago it was loaded.

The guns I worked on in the 70s were loaded in the days of the Civil War or "Indian wars" and never unloaded. They may as well have been loaded the day before I got them. ALL (as in 100%) were still 100% capable of firing.