I've read online that nitrogen gas suicide can be painful, I've specifically read that people who've seen it used on prisoners noted that they'd scream or groan in the last seconds of their lives. How true is this? I thought it was painless? and how long does it last for anyway
I think it's painful if you prolong the process. If you have a huge amount of Nitrogen coming in quickly (with Oxygen depleted from your body), you'll LOC quickly. That said, it might not look like a pretty death. There could be convulsions and mucus spewing out from you. I tried two test runs without following proper protocol (i.e. I didn't inflate the exit bag on top of my head first, hyperventilate, then take two deep breaths), but just putting my head into the exit bag with the air already scrunched out, letting Nitrogen into the bag, and tightening the bag. I had two very different experiences.
The first one with a lot of Nitrogen (my Argon regulator indicated 17 LPM), I felt these sensations almost all at once although I think it followed this order: ringing sound, felt like I needed spit/puke, dizzy, disoriented, and feeling faint. I'm pretty sure I was on the verge of LOC cause when I took the exit bag off, I knocked over one of my cylinders, but I didn't really register this when I had the exit bag on. This happened within a span of over a minute I think or a couple minutes. When I took the exit bag off, I was still disoriented cause I even forgot to close the cylinder valve before letting out the gas from the regulator and unscrewing the regulator from the cylinder valve. The side effect or after effect felt like overconsuming Cannabis edibles in the sense that you're dizzy, disoriented, maybe needing to puke, and you feel like you're losing volitional control. These nasty side effects cleared up pretty quickly though, although the disoriented state felt lingering for about a good hour. If I had done the process properly with the inflation of the bag on top of my head and hyperventilating, I might have lost consciousness (don't know about CTB-ing though cause I might have convulsed and knocked the equipment around).
The second one with less Nitrogen (my Argon regulator indicated 13 LPM), I felt different sensations. Again, I didn't do the proper protocol but just scrunching the air out of the exit bag, putting the exit bag on, and breathing in the Nitrogen that was coming in. This time I felt a sense of breathelessness, my heart rate spiked really fast, and I felt lightheaded. These sensations were less tolerable and I don't imagine I could've CTB-ed this way, as it would have been painful to continue, as LOC didn't seem to be quick.
All in all, I would say that the method becomes less painful if you have a lot of Nitrogen coming in and depleting as much Oxygen from your body as soon as possible (you can get that from hyperventilating beforehand), so that you lose consciousness quickly. You could see this was a major issue with Kenneth Smith because he held his breath, didn't hyperventilate, and didn't take deep breaths of Nitrogen, which meant he had a lot of residual Oxygen, so he didn't LOC quickly. I don't know if it could be defined as painful when he started expelling mucus and trashing about, as he might not have been conscious for that. That part isn't pretty.
I'm not going to say this is a painless method and a pretty method (you can see images of mucus discharge from several Helium and Nitrogen deaths), but the pain will be quick if the conditions are perfect (i.e. using proper protocol). The idea behind this method (like some other methods) is introducing hypoxia for death. As hypoxia increases, the more painful it becomes. That said, when it reaches to the point of LOC, you won't register the pain, so if you can quickly get to a hypoxic state wherein you LOC, it would be less painful than being in a highly hypoxic state and being conscious. I see the protocol as being getting from A to Z. You don't want to be lingering around in G, H, I, etc. cause those are painful experiences. A is painless cause you have Oxygen, but Z is also painless cause even though you have no Oxygen, you're no longer conscious.