Is she correct in thinking that large capacity magazines are in "common usage" and therefore these bans are unconstitutional?
I agree with that view, so she must be correct.
Seriously, what two firearms are arguably the most common in common use today? Let's begin with where most of the legislatures (other than NY State) are going, by defining "large capacity" as capable of holding more than ten rounds. What handgun is perhaps the most popular/most ubiquitous in the U.S. today (considering both police and private citizen ownership and use)? Probably some model of Glock. What are the magazine capacities of the various Glock models?
For rifles, I think the most popular rifle without question is going to be an AR-15 with a 30-round magazine. There is probably one of those, with multiple 30-round magazines, in the trunk of nearly every police patrol vehicle in the U.S. Almost any private citizen who owns an AR owns multiple 30-round mags. They're the "sweet spot." 10-rounders and 20-rounders are too small, 40-rounders and larger are unreliable.
So it's absolutely a valid argument, and based on Mr. Justice Scalia's language in Heller
the argument should be persuasive. Whether or not it will be remains to be seen.