Pages 44, 52, and 65.
And certainly, the difference between an AR-15 an an M-16 is less than a buck - the semi-auto version is a bit more expensive to produce, by a few pennies.
I have to think about your common use argument - while I like it, I am not sure that the court would buy it.
The authors are obligated to treat the arguments in Heller without requiring the overturning of Miller if it is possible to do so without losing the force of their argument. They hold out a "carrot" to the Court, suggesting that a later court might find a way to uphold the regulation of machineguns. It's not for this court to determine whether machineguns meet the Miller test. It is sufficient that handguns do.
On page 65 I see a statement that Congress has the power to regulate guns in DC. That is true, but it understates the challenge of meeting "strict scrutiny" for which the brief argues.
I don't see these statements as in any way reducing the force of future arguments to overturn the machine gun ban. Nor do I see anything that would aid the liberals on the Court in undermining our Second Amendment rights.