RD, I'd say the biggest objection would come from the overwhelming majority of gun owners who actually are responsible, safe, and not interested in spree killing. According to
this website (first hit when I googled "how many guns in the us"), there are an estimated 200
million privately owned firearms in the US, owned by 1 in 4 of the US population. Now ask yourself a few questions: "How many magazines must there be out there? How many of these spree killings happen, and how often are they happening?" If they are literally happening every other day, perhaps then a magazine capacity limit might be worthwhile.
But they aren't happening all that often, are they? Are you going to buy back all these weapons and/or magazines to get them out of the hands of law-abiding people? . I don't see too many felons turning in their guns in the first place, what's another law to ignore? So your magazine capacity limit punishes the responsible gun owners for the misuses of a small minority.
As to practical reasons, well, let's look at they actual magazine limitation that Clinton signed into law with the Brady Act. New production magazines were capacity limited. However, all the previously manufactured magazines before the ban were grandfathered in, and perfectly legal to own, buy, sell, and repair. While the "assault weapons ban" also limited new production weapons from having essentially cosmetic features, no limits were placed upon "compliant" firearms that used the same ammunition as the banned "assault weapons". So all a spree shooter with no grandfathered weapons or magazines would have to do is carry more "legal" magazines and change magazines more often, but the devestation caused would essentially be the same.
Let's look at places where private gun ownership has been largely eliminated, such as the UK. The 7-7 bombings there were orchestrated by people who were determined to kill as many people as they could. So they used homemade explosives derived from hydrogen peroxide. Hey, wait a minute, that guy in Aurora made homemade explosives too, right?
Determined criminals will find a way to misuse whatever they can get their hands on. I don't see how gun control will have any sort of appreciable effect as gun control advocates envision it.