The question the judge asked him was about a gun designed to avoid metal detectors. Whether there is such a gun now or not, the right answer was to say that he was not contending that such a gun was constitutionally protected.
Gura had to proceed carefully and allow the Justices to reach a correct understanding on their own. But he was too cautious. I believe Kennedy jumped in an one point and said that MG's were precisely guns that would be protected and that Gura's stance on the operative clause was too narrow. (I paraphrase.)
I know it's stupid, but a strong answer could be perceived as "smart-ass" and THAT loses the argument.
After I listened to the recast, I though Clement(!) was a stronger proponent of the operative clause than Gura. And that was a supreme disappointment.