...someone like Piers Morgan may say that 5 less rounds would reduce victims of gun violence by 50%.
They could, but they would be hard pressed to back it up with facts.
In an offensive situation, at least to some extent, the shooter can plan his reloads as Cho did in his shooting at VA Tech. He would go to the door of a classroom and shoot from the door until his gun ran dry. Then he would withdraw to reload, backing up his empty gun with a second one that was loaded in case anyone tried to exit the room. When reloaded, he would resume.
In a defensive encounter, one doesn't have quite the same luxury because it's not generally possible to withdraw to safety to reload--especially with multiple armed attackers.
Cho, killed more college students using only 10 round mags than Lanza was able to kill elementary students using full-capacity magazines.
Anyway, the point wasn't that the calculation results would end the debate once and for all, the point was to provide a reasoned response when people ask the question about why a law-abiding person could possibly need more than 10 rounds.
The argument will never be cosed because it's not always about the facts. Still, it never hurts to be able to provide a sound response when a question is asked by the opposition.