Something that is almost never brought up is those 40x46 low velocity grenades have an arming distance for safety reasons. Most of them are set for 10-30 meters before the grenade actually arms. There's a few HE ones that have an arming distance of 3 meters, but they're rare.
As for what happens when you shoot a 40mm LV round before its arming distance... you just hit them with an inert 40mm projectile. It would certainly hurt like fuck and potentially kill by sheer force of getting smacked with it if hit in the right place, but it won't turn your target into chunky salsa unless it's the couple of ones that do have a 3m arming distance. (I remember reading a collection of stories from Vietnam veterans one time and a guy was actually on patrol and shot at a VC with an M79 and it just hit the guy, surprised and scared the shit out of both of them and the rest of the guys in each other's patrol then started firing)
In which case, you too, might be caught in it.