Marshall Luton doing speedy reloads and hitting multiple targets. More at: Removable Magazines
Here's Marshall Luton hitting multiple targets with tactical reloads to illustrate the point. Imagine trying to rush him while he's reloading (each ring, is him hitting a different target).
And while he is very fast, and professional, this is a skill that many people practice, and can do well.
These guys are using pistols, but rifles are kind of easier. Also remember that most shooters have a backup gun. If someone charges, they drop the primary, pull the secondary, and ventilate them. Those that think magazine limits can help are ignorant or liars.
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Very fast reloads... whether you have to change every 10 or 30 rounds doesn't change the implausibility of charging someone while "empty".