If someone can explain this to me so that I understand it, that’d be amazing! Post comments below if you can.
http://rugerforum.net/reloading/7566-ammo-momentum-energy-chart.html
http://www.batesville.k12.in.us/physics/phynet/mechanics/energy/KENOTMomentum.html
In short, momentum is how much “oomph” a bullet carries – it’s ability to knock things over, etc. The momentum of the bullet equals the momentum given to the firearm as recoil.
Energy is how much work the bullet can do to the target, usually in terms of destruction. More may be better, but how it is used makes a big difference – will the bullet just destroy the surface of the target, spend it on the inside of the target or just go through and make life difficult for whatever is behind it?
Both are important. You want a bullet to have sufficient momentum to carry it through any intermediate obstacles (tough hide, ribs for game, etc.) but not so much as to go through as mentioned above. You also don’t want more momentum than needed as the resulting recoil can make shooting miserable in extreme cases.
Higher energies help expanding bullets do their job. Energy is mostly a question of how much powder and barrel length is used; momentum depends on that and bullet weight.
With the huge variation of targets, firearms and shooters, that leaves a huge range of applicable calibers and cartridges. But that’s part of the fun of our hobby!