There's a specific reference in the book (YS- can't seem to find it right now though) to special attacks, like tasers and the like, that work like a weapon attack, but their weapon rating (and weapon rating only) goes to creating an aspect. Bonus from accuracy still goes to stress (because getting shot in the chest with a taser will knock you out... getting shot in the face with a taser will knock you out AND hurt). Far as I can tell, the rule is more or less an extension of Overflow from YS214.
This might be the most appropriate choice for what you want to do with flash-bangs, since they can still cause lasting injuries if you're close to them when they go off (overpressure popping eardrums, burns from the magnesium flare portion, etc). Make the weapon rating good enough to affect most anyone who's not able to take cover AND look away, then 1 higher to make it sticky/persistent for a single use. Accuracy allows you to land it at someone's feet, which would cause stress as well as the effect.
Concussion grenades, btw, kill within 10 meters, almost as a given... definitely not stun material IRL. Video games got that one wrong. Stun grenades = flash-bangs. Concussion grenades = frag grenades without the shrapnel.
Tear gas... I'd run as pure maneuver. How long a given cloud lasts is so variable (dispersal due to open-ness of area, wind, etc) that letting extra shifts go to persistence seems appropriate.