Your general argument that Heller leaves some wiggle room is not unreasonable. However, I believe you are off base in claiming that AR-type rifles are not in common use. They are by far the most popular rifle sold in the US ( the manufacturers use the term "modern sporting rifle"). More importantly, they are the home defense weapon of choice.
Scalia's dismissal of the vague prefatory phrase in the Second Amendment seems correct to me on two grounds. One, such explanatory language does not limit subsequent clauses under standard rules of statutory or contract interpretation. If the Drafters had intended the Amendment to expire if those conditions no longer applied, they would certainly have included specific language to that effect. Two, the grant of the right to keep and bear arms is not made to militias but to "the people", a term of art in the Constitution that refers to the entire populace.
The Heller decision announced a broad right possessed by "the people". As with any Constitutional right, limitations on it are to be judged on a case by case basis. However, the burden of upholding limitations falls on those imposing them and the bar is high. This differs from the standard of normal scrutiny under the Equal Protection Clause, where government need only assert a rational basis. To bar an entire class of commonly used firearms would require an extraordinary showing of compelling need and a lack of less restrictive alternatives.