That is only 4.3 foot pounds and is insufficient for anything further than 5 to 7 yards to kill any game. I would say it is way underpowered for a .177 on rabbit. If you are accurate with it, close to your target (no more than 5 yards), and if it is quiet, then maybe you could get a shot without spooking or wounding the animal. I know it only take 3 foot pounds of energy to kill a squirrel with a head shot, a rabbit is likely similar, but I would avoid using the Optimus on anything larger than a sparrow, or starling, specially with the usual distances encountered to take rabbits without spooking them.