Common usage can be proven and argued fairly easily.
Really - I'll play devil's advocate here and imagine I'm arguing that AR-15's are not in common usage.
"Your honor there are 320 million US citizens. Lets assume that half are adults eligible to buy a firearm. Of those 160 million people
(NOTE: I'm guessing here) only 4.5 million own an AR-15. Your HONOR, that's only 2.8% of the people. That can hardly be called common usage".
That kind of argument could very easily sway an anti RKBA judge.
I guessed at the 4.5 mil. It's probably lower than that.