What's the point? The shooter would just re-load.
Reloading gives the targets a chance to flee or subdue the attacker. Rather than have capacity be defined in terms of the number of rounds, I'd prefer to see it expressed in terms of volume or the like. As others have pointed out, for 9mm pistols with grip magazines, having more than 10 rounds is fairly standard. Basing the limit on something other than simply the number of rounds makes it possible to grandfather in pistols with normal sized magazines that have more than 10 rounds while limiting the size of magazines for weapons that use 5.56mm or other rifle ammo.
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