posted by gutMaybe because fear is a main component of terrorism, and maximal impact means using the gun so many have been conditioned to fear as a "weapon of war". There's some advantage to more ammo in a clip and easy exchange, but when you aren't encountering resistance it really doesn't matter. I think a standard glock can hold 16 rounds, and extended clips are available. An AR ban, which was already tried and failed once, really won't do shit.
Also, the guns being legally purchased is a red herring. Do you really believe there is a gun law that will stop someone willing to die committing a mass shooting from figuring out how to get a gun?
On a related note, I think a plot was just foiled involving an AK-47...I'm guessing it was not purchased legally.
They’re called magazines, not clips.