Remaining Energy

500 minimum for two legged critters 1000 for ethically hunting.

All depends on what you want it to do when it gets there. At 1000 meters, my 77's have about as much punch as the .32 under my left arm right now does at 30 feet. My 77 grainers go into six inch circles at 1000 meters though...sooooo....is that enough for what I'm aiming at? ;)

By doctrine it is around 800 ft lbs at terminal. I have to respectfully criticize your question however as I believe it only entails a desire for a singular answer, however if we are...and as the question is worded, we are, talking about lethality...then I'd have to add as well that there are many other factors that stack on to ft lbs of KE.

When shooting "soft targets" say a 200 lb man, you will be looking for a few end results. Hydrostatic shock, penetration and bullet design.

Hydrostatic shock will displace tissue, fluid and organs, as the energy is transferred or dumped into said tissue.

Penetration for the obvious wound cavitation, and to create fatal hemorrhage.

Lastly I said bullet design because you need to have realistic expectation of what bullet designs are intended for, or if you compromise, how they will perform. Sectional density will play a big factor in penetration.The higher the better. All things equal, bullet weight, diameter and B.C. will result in more energy at terminal. Expanded bullet diameter will assist in hydrostatic shock. Without good expansion limited shock will be created, typical with bullet designs like SMK's.

We used the 175gr m118LR in the military and the Blackhills MK262 for the MK12's, I think both are still used today. However there are logistical issues and rules about particular type of projectiles that can be used unfortunately and I will say because they are used in service does not mean they are the best on the market. BTHP generally have jacket separation in tissue and reduces weight retention thus effects all the other terminal performance, it does not truly expand, but the long round about point is, with the right shot placement and even flying subsonic it has a potential to be lethal, but there are products that work way better. Also say you put a center mass shot on our 200 lb man at 1000y with your .223 77gr bullet, it could drop him instantly, or it can just kind of slip in and suck for him but he could be in the fight for minutes, hours, or maybe walk away from the whole ordeal altogether. Human physiology plays a role when you are cutting it that close to the iffy. Use the same example for say a .300 win mag with hmm a 180gr federal fusion soft tip, or any of the ELD-X lines....it would be safe to say at 1000y, 800ft or more lbs of KE, center mass shot that he will be wasted.

There is more, but this is the a quick overview of some of the importance to maximize lethal terminal ballistics. Hope it helps.

In the early 50s the police here in Germany had been issued a .25ACP like ammo. Studies had proven, that this would be enough to neutralize a soft target.

During the communist terror campaign of the 70s our police had to learn, 9mm Luger is hardly enough.

(Both considered at short range. )

I recently found this publication: https://www.google.de/url?sa=t&rct=j&q=&esrc=s&source=web&cd=2&ved=0ahUKEwj-6dXq0r3VAhVDPVAKHWdvCYMQFgg5MAE&url=http%3A%2F%2Fwww.dtic.mil%2Fget-tr-doc%2Fpdf%3FAD%3DADA512331&usg=AFQjCNHA74gfai92OTRKZL-yjXHeDjMrZg

It is named "taking back the infantry half kilometer".

It points out all the parameters influencing the effect of a round hitting a "soft target".

What scares me: the U.S. Government knew in the 1920s that a .270 would be best. The Norwegians and the Swedes adopted the .270 - like 6,5x55 even in the 1890s. And even though that round had been developed here in Germany, both the U.S. and the German Empire opted for larger diameters, worse BC and more curvy trajectory... strange...