A: Never
I think it would be view as a reasonable regulation of arms. The black rifle, after all, has a very large stigma in our society.
I have to disagree on 2 counts,
First the Heller dicision made reference to "Arms in common usage" the AR is the top selling rifle in America today, manufactured by 8 or 9 companies I can think of, that would qualify as pretty common, (like fleas on a dog

)
Secondly, the Miller decision, back in the 30's had that stipulation (not sure of exact wording ) about "Suitable to Militia use " . What can be More suitable for militia use than Semi auto clones of military rifles.
Have to agree with TAB though ( sort of

Jury box and ballot box ), Many lawyers will get rich proving it.