As stated before its not just the speed of the arrow but also the weight that counts. Just making the blanket statement that 280 fps is all you need is misleading. For example a 320 grain arrow traveling 280 fps only has 56 lb of kinetic energy while a 400 grain arrow traveling 250 fps has 60 lbs of kinetic energy. Nothing wrong with speed but its just one part of the equation not the whole ball of wax. Also take into consideration the cutting diameter of the expandable you are planning to use. A head that opens up to 1 1/2 or 1 3/4 inches is going to require much more energy than one that opens up to a more modest 1 1/8 or 1 1/4 inches. Last but certainly not least is the design of the particular mech head you plan to use. They are not all the same! Some penetrate better than others. The two best penetrating mech heads I have encountered are the Rocket Steelhead an the Snyper by Rocky Mt. These two heads should be fine on deer sized game even fired from rigs generating kinetic energy down in the 50 to 55 lb range. I would not say this for any other mech head at this time.