And if the shooter simply takes a dozen pre-loaded clips with him?
In a dozen different guns, presumably? How many guns can you realistically strap on? Even then, you are limited to, say, 120 bullets. I could be wrong, but my understanding is that you can currently get that many bullets in four magazines, reducing the cost and increasing the convenience.
All of these measures being recommended are not going to stop the types of crimes that have been perpetrated. They are knee jerk reactions to horrific crimes.
I agree they won't stop these crimes. All you can hope for is to reduce the amount of damage a perpetrator can engender.