Since the city in which my story takes place isn't an all-out warzone they can't just call in an airstrike, so I was planning on having one of them toss a concussion grenade into the room they feel would be most occupied, cover all the exits and wait for the survivors to come running out. I need for the grenade to even the odds at least a good deal without bringing the building, or the surrounding buildings, down, and it (the building) can be made of as solid enough material needed to prevent that from happening (I'm willing to use the power of artistic liscence for that at least, to a point. If it really has to go, it really has to go). I chose a concussion grenade as opposed to a flash bang because I'm not sure that the effect would last long enough to create a significant advantage when the odds are so skewed.
So, the question: Is this plausible?
-If so, how many people would the grenade most likely kill / incapacitate? (The enemies are not expecting this attack / aren't aware of the protagonists' presence, so they probably won't toss it back)
-If not, what would you suggest? (It's about eight at night, in a fairly deserted section of my fictional city, and to avoid getting rediculous my characters have access to the more subtle end of military-grade munitions that can be transported without a great fuss and, preferably, can be bought on the black market. Time period is present)
I have done enough research on grenades to make me feel like a terrorist, but if anyone can suggest something to look into, I'm not opposed to doing more digging! My characters are supposed to know what they're talking about...the problem is that I have no real idea.
Thanks in advance!
EDIT: The opponents are for the most part untrained, without clear leadership, and not expecting an attack of the size of the one they will be recieving. They are armed, some with automatic weapons, although they have only a basic understanding of how they work. None of them are wearing body armor or anything of that sort. I picture the building they are in to be a small warehouse, two stories max.