I'm not expert on grenades; the Army didn't take me due to my hearing problem.
To my understanding, a flash-bang and a concussion grenade are two different things. A flash bang is loud and disorienting, a stun-weapon.
A concussion grenade COULD kill you or blow an arm off if it went off close enough, but it does not fragment and throw sharp bits of metal out up to 75 yards. It is for use when frag is too dangerous, or in confined spaces, IIRC.
I figure you should be able to buy flash-bangs at the gun store and treat it like any weapon.
Concussion grenades, probably ought to fall under Class III restrictions as they're potentially lethal and explosive.