How would you handle a "Delay Action"?

Again with my "how do you do X from game Y" question.

So Bobby is acting all agitated, and I think he may be ready to escalate into violence. I don't want to be the one to start the shooting, so I wanna wait and see what he does.

What's the move?

In "other" games, this might be using a Delay Action on my Initiative. Then pulling out my shooter once I see Bobby going for his gun.

But here?

Acting under fire? Or is this just Going aggro, but with some "conditions"?

I'm not sure I understand your character's goal here.  Are you waiting and then shooting them if they do anything? Or are you threatening them somehow?

I could see going aggro if stated like "If he goes for his gun I point mine and in his face and tell him to think twice."

On the other hand if you are just going to shoot him if he goes for his gun, I'm not sure there needs to be a roll.

It might not be a move at all, like you say. It might be just narrating, and waiting for the GM to narrate some more. The PC isn't really doing anything, he is waiting to see what Bobby does.

But isn't the rules then that the MC makes a Move?

MC: "So Bobby works himself into a rage, and suddenly screams "Fuck you, you fucking fucker" and draws his pistol, shooting hot lead at you. What do you do?"

I would bend either read a sitch or read a person a bit to the situation.
Read a person
   what does your character intend to do?
   how could I get your character to shoot me/not to shoot me ?

Yup, if you're just waiting, then you're waiting. If you're trying figure out what he's going to do, your reading something or someone.

I suppose topicstarter refers to something like "reaching for your gun split second before the enemy", like in cowboy flicks - and in Wil West hack this kind of move would be needed, I think.
Something like "when you reach for your gun split second before the enemy, roll +cool, on 10+ you get to shoot them first, on 7-9 choose between firing at the same time and diving for cover, on 0-6 your reflexes are not as good as they used to be"

Read a Sitch. Take +1 forward when acting on the info.

Besides, the reason the good guy waits before drawing his gun in a western is cuz hes the good guy. He had to shoot because the bad guy went for his gun.

Read a person/sitch is perfect. There's no initiative to worry about, just the story, and no one is limited to a specific turn order or anything, right? So if you're waiting and sizing up what's going on... there's a move for it already!

Yeah, you can't do that. I mean, you can read the guy to see if he's about to shoot you ('what do you intend to do?'), and then act on that -- but you can't create conditional statements, you can only react. Welcome to the Apocalypse.

Well. Sort of. There's still the often skipped question of how a PC reads another person. I would have thought it was perfectly within the intent of the move for a Read a person - "What do you intend to do?" to be answered: "His eyes narrow and you can see his hand twitch as he starts going for his gun."