system
Given the following post, respond as if you're commenting on Reddit:
human
[Mass Effect] Why are there no non-lethal or less-than-lethal weapon options on standard firearms?

I'd guess a really low power concussive shot would knock the wind out of someone and put them on their ass so they could be arrested. But it just seems like no one ever has a stunner or other weapon, not even C-Sec.
gpt
Administrative C-Sec here. There really isn't a 100% non lethal weapon and labelling them as such will encourage more careless use. 

A non lethal electric stunner from a weapon calibrated for humans will

* Perturb a Krogan
* Hurt but not disable a Turian
* Problably knock out a average sized human
* Problably knock out an Asari
* Problably seriously injure a Salarian
* kill a Hanar

Even to Turian, Asari and Humans so there is a slight chance due to other circumstances that the stunner will kill the target. Heart conditions, wet floors, differences in skin conductivity due to perspiration, or drugs. Concussive weapons have even more variance and risk property damage.

Given the risks and the probable odds that you'll be dealing with a chemically impaired angry krogan (I know racist, what ever) assigning only lethal arms and use them accordingly is best. Less to think about in confrontations and simpler protocol. Only draw your weapon when you mean to kill someone.