What's the meaning of “look down” this student had said?

Question

(0:00) https://youtu.be/Lo52BObqCds

KASKY: Senator Rubio, it's hard to look at you and not look down a barrel of an AR-15 and not look at Nicholas Cruz, but the point is you're here and there some people who are not.

This video is about the town hall meeting held after the Florida School shooting. And I don't know what "look down" means here, so I'm having a difficulty understanding a whole sentence. I looked up the definition of "look down on" which means to think that you are better than someone, but I think it definitely doesn't fit this context.

What's the meaning of the sentence?

For those who don't know, Nicholas Cruz is the criminal and AR-15 is a gun he used.

Answer 1

In this context, look down is not a phrasal verb. The verb is look, and where you look is down the barrel.

look down the barrel of a gun has two common meanings. The most common use is to examine the inside of the barrel, to check that it is clean and undamaged.

It is said that all firearms record their history and perhaps this is the reason people look down the barrel of a firearm. An experienced eye can tell the method of cleaning, make a close guess about the number of shots put through the barrel, and understand the gun maintenance routine applied to the firearm. The gun digest

The second meaning is when you line up your eye with the back of the gun barrel and then align the front of the barrel with something that you want to shoot at: It is a primitive way of aiming the gun.

You are going to look down the barrel at the target. No greater king: a story of slavery and war

It's not clear exactly what Kasky is trying to suggest, but it's clear that the meaning is something to do with aiming an AR-15.