If you're forced to use your gun against an attack it's plausible (though in private sector self defense not likely) that you might run out of ammunition before that attack has finished, and need to get more into your gun.
...The reality is that finding cases where a private citizen needed to reload in the midst of an attack is tough to do; in the vast, overwhelming number of incidents people have solved their problems with the ammunition that was in their gun to start.
...If the gun itself is capable of dealing with the situation, having double that capacity allows you to compensate for any magazine or ammunition-related failures, as well as giving you the ability to handle those situations that may fall outside the expected.
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