Shotgun.
The thing about firearms is that... comparing it to video-game enemies, it's like a metal slime.
Knowing there's no going back when you pull the trigger can make it harder to do, but at the same time SI can't peak to make you back out afterwards. If I'm unsure if I want to ctb enough that I can't turn off the lightswitch, then I'd know it's not the time.
It goes without saying that si peaks after the first step of ctb. Jumping off, cutting wrists, ingesting poison, standing afront a train, etc. Firearms skip over that problem entirely.