Heh, I agree, to further expound....
Many cartridges have historically been developed to take advantage of an advance in powder technology. When we look back at the classic Holland and Holland cartridges we see lots of case volume to deal with the fast burning propellants of the day. The 308 Win was developed to take away the 1 centimeter gap between the top of the powder column and the base of the bullet in the 30-06 and make a cartridge of the same performance easier to use in machine guns...
The trend towards "short and fat" has been going on as internal ballistics evolve ever towards the ideal sphere of gas pressure generation.
So picking a powder based solely on energy density is a poor way to pick a powder (and the powder itself could be cut in such a manner that you couldn't make good use of all that energy in the first place). External ballistics don't care about powder chemistry, only final position of the muzzle according to barrel harmonics and velocity. Accuracy comes from precision, which is generally found in old fashioned single base extruded powders (for rifle).
Machine guns are awesome until you have to carry one.