In Calatrava's defense, I will admit, when I first started reading about this, I did think to myself, "Huh. I wonder if this means that a lower capacity magazine would've resulted in fewer deaths, here?" I think that's a reasonable question to ask.
As I've outlined above (and I don't know if you've been reading me, Calatrava), I don't think it's actually the case here that if he'd been held to "normal capacity" (or even "small capacity") magazines, it would've saved lives, even this case.
However, if you continue to think that, and if you also are unmoved by arguments that we have a right to such magazines, and are only concerned about net harm, consider:
Mr. Loughner, in his reprehensible action, killed six and wounded fourteen (I believe). This gives him a hit rate of 60% (if we assume one shot per victim, which may not be correct). So, if he'd had a ten-round magazine, and had been stopped during reload (which I think is unlikely, since I think a magazine malfunction contributed to his being stopped), AND we assume that he had the same wounded:dead ratio, we'd be looking at a casualty count of 2 dead and 4 wounded. Certainly, a less horrible outcome than reality.
Now, between 1987, and 1990, David McDowall's study on defensive gun use indicated that guns were used 64,615 times annually in defense against assault, robbery or rape. This number has been criticized as being far to few (others put the number literally in the millions). But, let's run with it - the real number certainly isn't less
The same study concludes shots were fired in 28% of those cases - roughly 18,000 times per year. Unfortunately, there is no data available that I'm aware of on average shots fired in these cases. If you look at (e.g.), the number of shots fired per Member of Service (MOS) from NYPD:
In 2000, you had 6.9 shots/MOS, and 16.8 shots/incident. I'd think the latter number is more interesting for this case (since you probably don't have armed backup). But, let's be conservative and use even 6 shots as "average". Under any reasonable distribution, I'd expect the right-most part of that curve to be over ten.
A Law Enforcement Officer earlier in the thread said he'd never spoken to a CCW holder who wished he had eleven shots when he only had ten. That's because CCW holders who wish they had an extra shot are called murder victims
In what percentage of those 18,000 "shots fired" gun uses was it needed to fire more than ten shots? 10%? 1%? Even with 1%, you still have 180 "good guys" who need that extra shot per year
, balanced against the 4 extra dead and 10 extra wounded in this incident. This incident, I'll note, is the only one I'm aware of in all of history in which the magazine size even conceivably might have mattered.
Even from a "net harm" perspective, I think the rational thing is still to continue to allow the good guys to have high capacity magazines. That's of course, not even reiterating all the freedom-loving reasons why it's a good idea.