Since most deer only measure 18" or less from the top of the back to the bottom of the belly in the lung area the vital zone is far less than 18" .In reality you should be able to place your bullet into a target area about half that size in order to make quick clean kills.I limit myself to taking shots at game only at ranges that I practise on a regular basis and only then if a solid rest is available and the wind is weak or in a direction that will have little effect on the bullet path.I also use a laser rangefinder to verify the actual distance as it is easy to misjudge.With that being said I regularly take deer and other game at ranges out to 300 yards and will under the right conditions shoot out 400 and in perfect conditions out to 500 yards.Many people are quick to criticize a person that takes long range shots but the irony is that most( but certainly not all) of those criticizing seldom practise and are poor shooters themselves.Many of them shoot larger groups at 200 yards than I and several others at the local club do at 500 yards yet they judge all others by their own limitations.If everyone adopted the practise of never shooting at an animal under any conditions or ranges that they don' t regularly practise at the range there would be far less missed or wounded animals.