Are you talking about movies? Cause we all know how 100% realistic those are......
But bottom line.....No, you cannot use a musket under the rain. Unless it's just a thin morning mist maybe...Or if your inside a building and completly dry. If it JUST started raining you might be able to fire a few shots, but once you and your cartridges get wet, party is over.
From Wikipedia: Gunpowder
Black powder arms must be well cleaned both inside and out to remove the residue. The matchlock musket or pistol (an early gun ignition system), as well as the flintlock would often be unusable in wet weather, due to powder in the pan being exposed and dampened. Because of this unreliability, soldiers carrying muskets, known as musketeers, were armed with additional weapons such as swords or pikes. The bayonet was developed to allow the musket to be used as a pike, thus eliminating the need for the soldier to carry a secondary weapon.
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