Ask: “Quickie divorce? Good.”
I went ahead and answered that I’d look over my shoulder because that seems like the most likely rational thing I’d do.
Ultimately, it would depend on the situation. If we’d just been in a fight, I’d be more likely to duck. If we were investigating strange sounds in the house, I’d definitely look behind me first. If it was purely out of the blue, freezing in confusion is most likely. Self-defense is the only option that wouldn’t happen under any circumstances.
I’ve always stressed it to be suicide/murder.
As to the OP, I’d freeze in confusion.
This right here. His rifle-barreled shotgun, used for hunting, is tucked away and the ammo is stored separately. So I’d be freezing and WTFing.
Besides (running with the hypothetical), if I tried to duck/dodge, how would I know that I wasn’t going into the path of the Big Bad that appeared behind me and getting myself shot accidentally?
I’d get out from in front of it. Not because I don’t trust my wife, but because that is my natural reaction when the barrel of a gun swings towards me. I grew up with too many people who never learned to respect guns (or the uselessness of “but it’s not loaded!”) to stay in front in a gun no matter who is pointing it at me.
For me, it’s got nothing to do with trust. First of all, as Asimovian said, it would be such a distortion of my world view that the question of whether my spouse was actually aiming at me would never enter my mind. The only thought I’d be capable of having would be “?!” So yeah, I’d freeze. I’d be as nonplussed as if he was suddenly riding a tiger, or suddenly speaking fluent Swahili. And even after I had a moment to process that, yes, it’s not a dream, he’s really standing there with a real gun pointed at me, my very next thought would be, “…so apparently, he’s gone insane.” And at that point, I’d duck.
But the thing is, let’s say for the sake of discussion that he owns guns and is comfortable using them, so the sight of him holding one and aiming it is not bordering on the impossible, and that I trust him more than I trust myself, so that the idea that he’s actually gunning for me is inconceivable. And let’s also assume that I’ve got cat-like reflexes so that I wouldn’t just freeze at the realization that This Is Not A Drill. Even then, if he’s aiming at someone or something so precisely behind me that he has to aim through me? I’m not wasting precious time checking to see what it might be; I’m going to get the hell out of the way and let him shoot it.
While I answered that I’d look over my shoulder, really I’d freeze first, confident that there was something behind me I don’t really want to see. I’d try very hard not to move and ruin his shot, but it wouldn’t be very long before I wouldn’t be able to help myself. I’d look, and probably do something very stupid that would put us both in more danger.
Pretty much this.
We not only think of these things, we talk about them, we practice what to do if an intruder comes from there or from the back, etc…
She would not simply point a weapon at me, even if bad stuff was sneaking up on me. That is not how it is done, well, except in movies.
We have both been married before, paid the lawyers, etc… No more. Joint decision: Both old, tired, won’t play that game, already made most of the mistakes so if it absolutely that we could no longer live together, we would try it living apart …
then if we for some reason we thought even that would not work, we are going out front & have a quick draw contest. It will be much cheaper, the survivor will get all the stuff ( not enough to fight over ) & easy to get away with because who could claim that it was not self defense. We both have CCW & are always armed.
She is meaner but I am a better shot.
Works for us.
So I picked door #4. 
I would do a combo of freezing and ducking. I would duck and freeze in confusion.
I voted for “duck,” as I think that would be my split-second response to seeing a gun pointed at me regardless of the person holding it.
But in that case the gun wouldn’t be pointed at me, right? Maybe it’s different for me because I’m short, but it’s hard to imagine any threat behind me that would be the same height as either my heart or my head. If I saw my SO pointing a gun in my direction but not at me, you bet I’d turn around.
If a gun were actually pointed at me I believe that I would freeze, not so much in confusion but in order to ensure that I did/said whatever the person aiming at me asked me to do. No sudden movements, etc. The priority would be not getting shot/killed: confusion can wait.
Yeah, that’s like Rule One. Never point the gun at anyone you don’t intend to shoot. I actually have trouble watching movies/shows with bad gun behavior now.
My spouse is retired military and as such I know he’s aware that you don’t point a gun at anything you don’t want to kill. Therefore I would assume he is confused or not in his right mind (and had apparently stolen a gun from somewhere) and I would duck/dive out of the way ASAP.
^^^This. I’m the designated killer of things.
After a flash of confusion I’d probably drop to the floor so she could either get a clear shot at what was behind me or lose her point of aim and only wound me. 
I couldn’t imagine her touching a gun, let alone aiming one at me. It would have to be some kind of alien mind control at work.
If I was your wife I’d probably learn to shoot just so I could wing you now and then.
I kid, I kid.
Duck for cover so as to not block her shot at whoever she’s aiming at behind me. If I take the time to look and see who that is, that’s probably too long of a time for me to be standing in the line of fire.
My girlfriend doesn’t know how to operate a firearm very well.
If she means me harm, ducking for cover is my best bet (and a pretty good one, she’s a terrible shot).
If she means something else harm, ducking for cover puts me away from where she’s aiming because she will likely miss and I don’t want to accidentally get shot.
Immediately step out of the line of fire, maneuver the gun away from me and into a safer direction and reprimand them for violating at least two of the four safety rules, then retain control of the firearm and end the shooting session we would no longer go shooting together until she learned proper muzzle and trigger discipline
I ticked the box for looking behind me, but actually I would assume it was a water pistol and start running and laughing. Because where on earth would he get a gun?
And it would so like him as well, to pull a really serious face and squirt me with water when I’m least expecting it. The other day he was apparently “practicing ninja moves”, and when he realised I hadn’t noticed he huffed: “a ninja could come in here and steal all our paintings and you wouldn’t even notice!” (I don’t know why ninjas would steal our paintings, they were painted by my sister.)
So I’ll change my answer to “other”: I would think it was a silly game. And it probably would be, since guns are rare here.
You’d be surprised how many people have suggested that.