Read up on the pros and cons of infantry rifles chambered for a full power or intermediate rifle cartridge. The gist of it is that in any given contact a very, very very small minority of shots hit their target therefore infantry should be equipped with something that fires the smallest, lightest cartridge that does the job so that they can carry more of them.
Magazine capacity reductions are easy to circumvent (a $30 stamp set can put a "pre-ban" date on your magazines) and most of the people doing mag dumps are either not subject to those laws (cops) or have no intention of following them in the first place (criminals).
The other problem is someone dressed and equipped like a fully armed infantryman showing up in a mall...gets noticed. You can't stealthily carry a ton of cartridges AND have them easily accessible. You'll have to put them in a backpack or the like, changing the equation.
Yes, if they play things perfectly, you will lose every time. The reality is, most of these guys are pretty average and make numerous mistakes. The guys who play it out perfectly never get caught regardless of the law. That isn't an argument we should make murder legal.
Magazine capacity reductions are easy to circumvent (a $30 stamp set can put a "pre-ban" date on your magazines) and most of the people doing mag dumps are either not subject to those laws (cops) or have no intention of following them in the first place (criminals).