So I've been hacking away at a rare npc for a little while now, just got its shields below 20%, and only recently has the % stability started dropping...but it appears to be dropping faster than the npc would die based on my rate of dmg...so does this mean if I keep attacking that I won't get to kill it even if the time remaining on it doesn't expire?

I can't find anything on the forums or wiki to explain what happens when an npc reaches 0% stability. Does the stability % start to rise again if I leave it alone for a little while?

I think the % stability on the battle tab just doesn't update well. You can rely on the information in the NPC's popup box though.

Yeah a couple of them the stability made sense as their total health %, but most of them it drops rapidly when they are about to die. I can't make sense of it. Only on boss NPC did it make sense.

just making sure it wasn't going to royally foul things up somehow, ty...I got quite lucky, first rare npc I've seen and I've never killed an npc before.

Nice. Yeah, even when I'm focusing on missions I make sure to check the tab, see if something pops up randomly. I plan to get 6 slots and then the Exotic Tactical Algorithm, so hopefully I'll run into some good ones while finally doing Decrypting the Vault.

Actually, a tip. As much as you may want to finish that, if you are starting to see crimson flamehawks, leaving mission NPC on seems to reduce their appearance. Right now I am leaving titancore pods on because the XP isn't bad vs. the shots taken. Having 6 slots + the algorithm is a very good thing, I'm working on the slots now, but what you do is wait for 4x flamehawk to expire, farm until you get 4 more, then purge them and continue. With 6 slots this lets you go on average 50% longer. You can burn quite a bit of energy on good XP that way.

Perhaps, but I want to get the scientists and the next mission, and I don't want them decreasing my chances of rarer NPCs either. I'm not worried about the experience so much as the rare drops.