Rangeball, because your bow STORES more than 70 pounds of energy as you draw it. You don' t just put 70 pounds of your muscle energy into drawing a 70 pound bow, especially one of the new cam designs that hits peak weight early and holds it until nearly full draw.

The longer the draw length, the more energy it stores. The lower the letoff, the more energy it stores. Someone with a short draw length and high letoff will likely never get 1 ft lb of energy per every pound of draw weight. Someone with a long draw and low letoff would likely get more than 1 ft lb for every pound of draw weight.

Mark, I' m sitting here looking at Norb Mullaney' s bow test report for the Browning Eclipse. At 30" draw, 60 pounds, 75% letoff, it stores 70.55 ft lbs. At 65% letoff, it stores 71.78 ft lbs. At 65% letoff, it shoots a 400 grain arrow at 244 fps for some 53 ft lbs of energy. It shoots a 500 gn arrow 222 fps for just under 55 ft lbs. It shoots a 600 gn arrow 207 fps for just over 57 ft lbs.

This bow, at 30" draw, NEVER hits 1 ft lb for every pound of draw weight. As you can see, increasing the arrow weight does make the bow more efficient, but at the cost of arrow speed. IF your bow showed the same KE gain as the Browning, 2 ft lbs for every 100 grains of arrow weight, then you might get 63 ft lbs of KE into your arrow by jumping weight up to 836 grains. Doubt you' d want to do that though.

I believe strongly in heavy arrows and momentum, and think KE is secondary in importance and highly overrated when it comes to penetration, but there is a limit to how much speed I' m willing to give up.