The most important reason to carry an extra magazine with a semi-auto is immediate action when it fails.
Dropping the magazine, clearing the pistol, and then inserting a new magazine is the quickest way to clear some malfunctions and get a semi-auto back in action when the basic tap-rack-bang approach fails.
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That's not a factor in a revolver, where the most common failures are both rare and preventable. Keeping your revolver clean, ensuring the hammer spring is strong enough to prevent light strikes, and ensuring your ammunition doesn't get longer under recoil and jam the cylinder takes care of 99.9% of the potential problems.
Dropping the magazine, clearing the pistol, and then inserting a new magazine is the quickest way to clear some malfunctions and get a semi-auto back in action when the basic tap-rack-bang approach fails.
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That's not a factor in a revolver, where the most common failures are both rare and preventable. Keeping your revolver clean, ensuring the hammer spring is strong enough to prevent light strikes, and ensuring your ammunition doesn't get longer under recoil and jam the cylinder takes care of 99.9% of the potential problems.