I wonder how many votes they could get for a gun control measure.
You have the 46 Democrats. Heitkamp voted No on the Manchin-Toomey bill and on the terror watch list bill, but I heard (at least for M-T) she switched her vote after realizing she wasn't the deciding vote. I think if it came down to her she'd do the right thing. Mark Kirk has also been a pretty reliable vote for this.
Republicans who voted Yes on
Manchin-Toomey: Collins, McCain, Toomey (duh), Kirk. That's 50.
I do wonder if fucking over a hundred people being killed or maimed in Orlando would change any votes. Sasse's presence at this filibuster is interesting - he's been positioning himself strongly against Trump and seems to be presenting himself as more moderate/reasonable, so I wonder if he could be persuaded. Portman came out in favor of barring people on the terror watch list, so maybe he'd agree to expanding background checks as well. That's 52. Beyond that...