OK here goes...
Why would I be carrying a snub? my instinct is tight groups and pulling the trigger as fast as I can, so there is no way I would be in this situation to begin with.
Ignoring that fact, lets continue. A shot to the back of the head would be good, assuming you have a clear backing, and are not risking anyone elses life. Even if the shot only buys you 1-2 seconds, that should be time enough to close the distance between you and him and grab his firearm.
BTW, since the other thread was closed, and I was chewing my tounge off wanting to say this... if a guy walks in with an AR and you go all rambo and flip the table, .223 FMJs are going to punch through that table like a paper plate. The only thing that flipping the table would do is to inform the shooter that you are prepared to have a shootout, thus making you target #1.
I do, indeed, look forward to your next "what if" post. Try to pick a better firearm in the next round, though.