Something else to consider: Souter was a Republican appointee who turned out to be quite liberal.
It's entirely possible that an Democratic appointee could end up being a friend of the 2A, or at least inclined to stick with the Constitution as written.
+2. Historically, SCOTUS justices have a habit of not turning out exactly the way their nominating president had hoped.
Secondly, in today's highly charged political climate, it's difficult for any
justice to make it through the Senate if they've made any sharply definitive rulings on a highly controversial "wedge issue", regardless of which way the ruling went. So far, the Obama administration has mostly tiptoed around the gun-control issue, so I expect we won't see any nominees with a strong anti-gun record. Time will tell, though.