What exactly are you seizing here, the upper hand?
It depends.
Okay, so I'm not Vincent, but in my reading, the "combat" moves (act under fire, go aggro, seize by force) are all written with the assumption that you actually have an objective you're trying to achieve, and the person you're shooting at (or who's shooting at you) happens to be preventing you from achieving that objective. All three of them focus first on getting something done, and only secondarily shooting people. Acting under fire obviously is for doing it without anybody getting hurt; going aggro is for scaring off or threatening people before having to kill them; and seizing by force is for when you'd really rather not have to talk to them any more.
So in most situations, as long as you keep that in mind, the system should work seamlessly. When you're in one of those rare situations where your number one priority really is just killing a guy, it's a little more confusing, but if you keep in mind the effects of the moves, things should still work out fine.
If the situation is that you and, uh, Plummer are standing across a table with handguns pointed at each other, and you say "Fuck it, I shoot him," what do I do as MC? In the majority of cases, I say it's seize by force, because a) "this is a move for when the guns and knives and crowbars are already out on both sides," p. 195, and b) seize by force is the move where both parties take harm, which seems appropriate. If you wanted to, you could say you're seizing the moment, the upper hand, his life, whatever, but honestly? You should probably just seize his gun. I'd probably say "You're seizing by force, what do you want to seize, his gun?" Alternately, you might seize the escape route, to keep him in the room -- depends on the situation. Even in a case of straight-up murder, there's generally SOME tactical advantage you can try to seize. (One other thing I could do as MC is just make it an MC move -- inflict harm as established or trade harm for harm -- but that, again, is pretty contextual.)
Why wouldn't this just be an act under fire? With the fire being the NPC's gunfire?
Well, it could be -- I don't really have enough information. A key design idea about Apocalypse World is that the moves are heavily dependent on the fictional detail; that makes bare-bones hypotheticals very difficult to answer. In an actual firefight, tables would be falling, people would be running, innocent bystanders would be dying horribly, and you, the PC, would presumably be DOING something, diving for cover or getting around the opponent's cover or digging in and calling for help or some sort of action that I could translate into acting under fire or whatever. If you just said "I stand there, where I am, and continue to fire?" Well, firstly, I'd be way more likely to call that trading harm for harm, as an MC, but if I were to pick a basic move, I'd probably pick seize by force again, and again ask you what you're actually trying to SEIZE. The other thing is, as above, if I was running the beginning of the gunfight you've already chosen something to seize and so you can't just be like "I stand there and shoot." By the time you're "engaged in a firefight" you, mechanically, have probably already identified a goal and taken steps towards it, and the scene can build around that goal.
I hope that's clearer!