The moment the Tachikoma states that the rifle was frustrated because it's user wasn't listening to it, Tatsuo goes back to the trajectory on the screen. And sure enough, there it is. The calculation isn't for the human user.

"It's self-targeting. The gun has an AI."

Oh, that is beyond unsettling. It's one thing for ballistics to have targeting assistance like heat-seeking. It's another thing entirely when the gun is the one literally calling the shots.