Grim Reaper's Sprint

 is a Fallout 3 perk and Fallout New Vegas perk

In Fallout 3, if you killed a target while in V.A.T.S. all of your action points are restored upon exiting V.A.T.S. mode. This was changed in Fallout: New Vegas as killing a target now restores 20 action points after exiting V.A.T.S.

Behind the scenes
The perk is a winner of Bethesda's Fallout anniversary contest, submitted by Marc-Andre Deslongchamps.

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Fallout New Vegas
In Fallout new Vegas the perk grim reapers sprint only gives back 20 ap on a kill in V.A.T.S. Although this is a significant nerf the perk is still very useful with high power, low ap cost weapons. For example, the holorifle from the dead money dlc has an ap cost of 30. Assume the holorifle always kills a deathclaw in 2 shots (criticals and sneak attacks ignored). In V.A.T.S that would total 60 ap, and in turn you would regain 20 ap resulting in an overall cost of 40 ap. This is a 33% reduction in ap cost, or in other words the courier gets to fire 4 shots per the normal 3. This perk is also even more effective agains less tough enemies. Assume the holorifle will always kill a raider in one shot. The ap cost would be 30 ap. But on the kill 20 ap is given back, resulting in a total ap cost of 10. This is a 66% reduced ap cost, or in other words the player gets 5 shots per the normal 3. For fans of high powered guns this perk is still very viable and recommended.