Are you sure about that? Stolen goods don’t often benefit thieves directly; they tend to have some sort of support network, even if their lone fence is just a guy who doesn’t ask questions. Besides that, I somewhat doubt that there is some code of conduct among criminals that prevents them from mobbing your house if they found a few more partners.
But that is a point I’m willing to set aside. So-called high ammo capacity has more important benefits than crowd control. One of them is that it often takes more than one shot to stop a threat, when it comes down to shooting. For one, people miss; it’s unavoidable that people have a good chance of missing when they shoot. For another, you have not necessarily stopped any threat if the target is shot only once. To stop a threat (in the sense of using firearms), you need to keep sending the lead in the same direction until it is stopped; if the idea is that reloading takes a bad guy off the target and puts him at a disadvantage, the same goes for legal usage from someone defending.
So let’s say an opponent in a firefight is using an illegal high capacity magazine and you are using the legally limited magazine, all other factors being equal. Who is at a disadvantage? This scenario may sound contrived to some, but it must be considered because the fact is that banning high capacity magazines will do very little to keep them off the black market. All a magazine has to have in most cases (no pun intended) is some sheet metal and a spring, neither of which is exactly hard to come by or handle. Not that the black market has trouble producing more complicated items like firearms themselves. (Actually, I digress, but don’t count the black market out on anything anymore.)
Ultimately, I approach my opinions to weapons legislation on the same consideration I would approach any legislation: law that does not concern itself with secondary as well as immediate effects is ill-considered. Putting the black market at a serious advantage relative to the legal market is one such secondary effect. We even have a nation-wide case study to look at. The AWB (1994–2004), which limited magazines for civilian use to ten rounds, had an effect only on legal firearm owners while it was in effect (reduced the supply of new such magazines, making them more expensive). In that time, plenty of criminals were found with plenty of magazines that stated right on them that only military and law enforcement were supposed to have them. But these sorts were already breaking a host of laws, so what’s one more to them? So while I’m all for nailing criminals harder (not that it really seems to deter crime), if your goal is to reduce senseless killing, this measure is not going to do anything good on its own. Confiscation would also be highly impractical—at the end of the AWB, there were still plenty of high-capacity magazines from before the ban (millions of them).
To summarize more abstractly:
If you accept that potentially lethal force via firearm is a responsible measure to defend yourself from likewise lethal force, there is no number of bullets in a magazine that makes it stop being responsible, because it’s the application of force that counts. In the meantime, you cannot
count on an assailant to limit his use of force or otherwise play by the rules, and laws (on their own) will do nothing to deter him due to logistical issues.