Their logic, although very flawed is pretty simple. He was stopped when he was changing "clips", therefore if there were smaller "clips" fewer would have been killed/shot before he had to reload. Typical approach.....
To extend that "logic" to its conclusion, nothing except a single shot firearm should be allowed. THat way you can stop a criminal after the first shot! Why 10 rounds? What's magic about that?
And these single shot firearms should ONLY be allowed because of that pesky second amendment.