Study kilimanjaro Bell and his elephants. He reliably got into the brain pan through a foot of skull with a particular formula with a 6.5x54 Mannlicher running 159/160 gr solids around 2400 fps or 2000 ft/lbs energy.
Then study the .700 Nitro Express, throwing 3/4" 1000 gr solids around 2000 fps or 9000 ft/lbs and it demonstrated not enough penetration to do the same thing and sometimes only knocked elephants out. Boddington has written about this to help your Google fu.
Then someone took a 416 to Alaska as a 'do all' rifle and learned it just poked holes in goats for very unimpressive performance as compared to a .270 win with a modern expansion type bullet that flattens them like lightning. It's just a good 'extremes' example to help connect the dots on this whole energy debate.
And to further connect the dots there ARE numbers that you CAN use to help better predict one choice over another...but ft/lbs is NOT it. Energy is a useless measure.
So when you break all this down you realize that FOR GAME INTENDED you need appropriate IMPACT VELOCITY, SECTIONAL DENSITY, and BULLET CONSTRUCTION. Those 3 measures are the best way to look at things now.
Yes our sectional density deteriorates at impact as we use expanding bullets so you need to understand the construction aspect to the impact velocity aspect to the game intended. There is a sliding scale between construction type to impact velocity to penetration potentials that you can line up for your goals, distances, and game intended.
Study the bullets first. You can push low SD bullets of rapid controlled expansion really fast and they have terrible penetration, which is good for smaller game explosiveness. You can push high sd rapid controlled expansion bullets at moderate velocities and they penetrate really well. You can push high sd delayed controlled expansion bullets at moderate velocities that will penetrate very deeply and maybe deeper than that same formula at high velocity. You need to look at the bullets and impacts velocities you know, learn those numbers, then you can apply anything new in comparison to that but you'll be doing yourself no good if you use energy in your factoring.
Just to solidify the above, the Mannlicher/Nitro Express elephants example. The formula that explains why the wimpy mannlicher could do the job and the nitro express is a lottery is this. One of them matched impact velocity, sectional density, and construction better...for the game intended. The mannlicher has SD of .328 and 2400 fps launch (impact velocity slightly less of course), and construction was solid which was appropriate for game intended as penetration trumps all else in that game. The nitro express only had .292 SD and 2000 fps launch but with appropriate construction for game intended. So the Nitro Express did not have enough velocity to overcome the lesser SD and proved itself a liability.
When you study this further and further you come to realize why the 6.5's in general are so popular, they have unusually high sd heavy for cal bullets as compared to most of the standards we've always used. You can study the 375 h&h options and why it's considered an African standard, then apply to things here for whatever your goals might be.
Enjoy the process but drop the energy thing. You need 3 other things that need to line up for game intended that are far more important.
I applied this formula and shoot something wildly different now than when I started. Batting 1000 over 5 seasons and 15 animals with something most would consider too small or weak for the job. Most guys just shoot something common and are generally covered if they stay away from the varmint bullets, as most things work most of the time, but truly lack the understanding of it all and could care less to learn it...better things to do. And that's just fine, for us ballistic nerds we find fun in learning about this stuff, just passing it along.
