California and other states try to make the argument that large capacity magazines, AR style firearms etc... are "not in common use" and therefore not subject to 2A protection. Any fact to the contrary undermines that argument and kicks out a leg in their arguments to ban or regulate them.
EDIT: This would be funny if it weren't used against us. California passes laws to ban something, then argues that they are rare and can therefore be banned.
From the judgement:
"To the extent that magazines holding more than 10 rounds may be less common
within California, it would likely be the result of the State long criminalizing the buying,
selling, importing, and manufacturing of these magazines. Saying that large capacity
magazines are uncommon because they have been banned for so long is something of a
tautology. It cannot be used as constitutional support for further banning. See Friedman
v. City of Highland Park, Illinois, 784 F.3d 406, 409 (7th Cir. 2015) (“Yet it would be
absurd to say that the reason why a particular weapon can be banned is that there is a
statute banning it, so that it isn’t commonly used. A law’s existence can’t be the source
of its own constitutional validity.”)."
)
