Sylvan's right, there's a ceiling on what a bow can do, because there's only so much potential energy stored in the device at full-draw. As he explained, 100% efficiency is impossible, but that is the limit on the bow's output.

However, if you could design a cam that would allow you to draw TWICE for one shot, you could, in theory, store 2x118 foot/pounds of energy, and effectively nearly DOUBLE the output.

Think about a pump-action BB gun. The airtank housed inside the airgun stores energy without releasing it until the trigger is pulled. If you pump it once, you might get 100 f.p.s. If you put two pumps in, and uncork it, you'll get maybe 200 f.p.s. - the same would apply to bows.

So where I'm going with this, is if you could engineer a cam that would allow you to draw the bow several times to "store energy" and unleash it all with one shot, you could overcome the 400 fps "ceiling." I'm thinking something along the lines ofa "ratcheting" cam that would compress the limbs repeatedly, and unleash the fury all at once.

I shouldn't think out loud, because it makes me sound like a total idiot.