I don't think it was a tactic. I think she is genuinely scared that machine guns are to be protected. Liberals know what they want but they very often also know what the rules say.
For example, Hillary wouldn't want Florida and Michigan to count for the Democrat nomination if Obama had won the votes. She knows full well what the rules were. She just wants the rules to be ignored so that she can have what she wants.
The same is true of Ginsberg. She knows what she wants but she also understands that the rules, at least as they are unfolding, are not going to let her have it. The challenge for Roberts will be to convince her that she can side with "individual right" and "strict scrutiny" and yet those nasty, icky machine guns (that nobody needs except police sargeants in L.A.) won't necessarily be protected.
You’ve described libs perfectly -
rules be damned, I want my feel-good outcome!