First off, I have fired 720 gr bullets at 1800 fps in a nine pound rifle, and I never want to do it again. I've also fired bolt action 50 bmg rifles in comfort, but they had artillery-style muzzle brakes and weighed over 20 pounds. Neither would make a pleasing hunting rifle, in my opinion.

The choice of the constant by which we divide in the energy calculation differs, depending on your source. The constant is used to convert from grains to pounds (a measure of weight), and simultaneously from weight to mass. The discrepancy is attributed to the gravitational acceleration due to gravity, which varies depending on location on the surface of Earth. The commonly used “average” acceleration is g = 32.1737 ft/s^2, for a constant of 450,432 (the formula is E = (1/2)mv^2, so the constant is 2 x 7000 x 32.1737). But ammunition makers often use 32.163 f/s^2 (giving a constant of 450,282). I have seen others (such as 450,420, 450,425, and 450,240). I would be hard-pressed to say any particular one is correct and some other is wrong.