I maintain the sweep is entirely useless, unless you are in a very special situation where there's a barrel you want to hit in the same turn as a particular enemy, and that barrel can be hit with the sweep (hard to tell, because the precise trajectory of the sweeping bullets isn't easy to predict.)
A single bullet from the chaingun cannot kill anything - not even a former human. And the sweep, unless you get a bugged one where it goes back and forth, will only shoot one bullet for a particular trajectory. In the end it's better to re-start your chainfire at the new angle if the change in angle is great.
I disagree. Continuing to chainfire with the sweep you'll miss a couple of bullets, but then you get the full chainfire again next turn. If you stop chainfire and start again you have to do the "start" and "warm" shots again, and end up shooting less bullets overall than if you continued with the sweep.
Shooting an unmodded chaingun with chainfire looks like this:
3-4-6-6-6-6-6 (37 bullets)
If you stop sometime in the middle and start again it'll look like:
3-4-6-3-4-6-6 (32 bullets)
You lose 5 shots when you stop in the middle of your chainfiring. I don't know how the sweep mechanism works, but I doubt that even with a wide sweep arc you'd end up losing more than 5 shots to the sweep effect, with any gun*. So it's better to keep on chainfiring, your number of shots fired at monsters is higher and the sweeping shots might end up hitting something.
There still might be two reasons to stop chainfiring:
1)Conserve ammo (if no ammochain, no bulk mods)
2)Don't want to hit a barrel in between monsters
*I haven't actually tested a triggerhappyx2 Minigun with a 180 degree sweep. Somebody might want to do that.