I can tell you that it’s used more for displacing enemies than killing them (much like in real life).
There has been a lot of misinformation going around that Tear Gas is useless. Why drink from the canteen when you're by an oasis? Besides you're going to need to save as many as possible for later waves. I still advise grabbing Armor between Waves. For the record I wrote a walkthrough for three of the Survivals before discovering this, would have been nice to know then. For example, if you use a Standard Armor when your meter is low it will refill a portion of the meter instead of completely replacing it like the old system. When you 'consume' Armor from the Inventory it will stack. Ranks 135 and above can carry the full ten.
If you are Rank 50 you can only carry four spare sets of Armor. As of Patch 1.13, you can now 'consume' Armor at any time from the Inventory screen.
