Grenades
I've seen some people rapidly throw grenades at like 3 Throws/Second. It could be a bug, maybe it isn't. Just wanted to put this here just in case it is a bug. I don't know how they do it, but maybe you can check the code for grenade throw
Re: Grenades
Ya there is a skill called "multi-grenade" for gunman that allows them to throw multiple grenades with one chuck. So unless there is something else going on, I think that's what you are seeing.