That you can't restrict something then later argue because that thing is so little used there's no reason to have it at all .
I would argue they could. You don't think say a ownership level in California and other jurisdictions progressively lower due to bans doesn't mean that at some point this accrues to them being less common in a national sense?And with registration as a specific class, in most states the only one required to be registered, the people who don't register would not count toward common use and the only "legitimate" registered ones do, nor would some 80% builds, which results in even a smaller number.
Is there a rejection by the courts of a concept of what is common if it is regulations making it less common? Would not most civilian AR-15 be select-fire if it was not NFA regulation? Has anyone successfully argued that select fire
would be common f not for regulations and therefore should be considered common? so how are regulations causing lack of commonness going to be excluded in consideration of commonness? The same could eventually be said for >10 round mags if enough states limit them and or federal govt does..
Secondly here is the poltical-cultural issue. Firearms ownership is inherently private. Yet the press, and even submitted material to courts are using GSS and GSS type face-to-face (the most certain way to get undercounts on any privacy question) numbers for ownership, what is owned, demographics of ownership, and numbers of weapons owned. Like the claim that 2% of Americans own half of US guns. and labeling GSS as the "gold standard" of surveys. This goes to a public perception as to what is common. This n turns affects policy and how many state and federal legislators will believe that AR-15 is not in common use whether it is true or not. would you tell a face to face interviewer who bangs on your door that you have an AR-15 in your residence?
thirdly by raising hassle, cost, and or stigma, and implying low ownership, prospective buyers may think owning a semi auto rifle is an outlier, abnormal, and many people do tend to want to be in the norm.