How would you make grenade and ring seperate during specific frame?

So I am making model of grenade which is done now, and now I am animating animations for it like throw deploy, idle etc…
I faced 1 problem now, I have a ring on grenade and during frame 40 left hand moves to it and grabs it but how do I make so that ring seperates from right hand and body of grenade and falls off or follows left arm ?

So far I have:

Hands (default css beacause easy to use them)
I have model of grenade animated and put into hands that are holding it and attached to bone of hand so grenade moves with right arm. And I have a ring made as seperate object with bone attached so that when I move that little bone ring moves but grenade doesn’t . So how would I make now so that ring follows right hand animations to up to frame #40 and falls of then?

Pic:

Well, is the pin parented by a bone? As far as i know, the animation for the HL2 nade doesn’t actually pull out the pin, the finger just hooks it, and quickly moves it off-screen… But if you want the pin to actually come out, add a floating bone to it, and make the bone match the movement of the hand that holds it.

Create a null helper and align it to the pin bone, parent it to the grenade bone.
Unparent your pin bone, then apply an animation constraint to it with the null as the source. use both position and rotation constraints.
under the motion tab, put a keyframe for the constraints at frame 39, at frame 40, make another frame with the constraints strength to 0.
Now the bone will be parented untill frame 40, then sever.

Now, I’m no modeller or anything like that, but wouldn’t he be able to parent the bone to the finger when it’s pulled off the grenade so he don’t have to bother with that movement too?