I'm gonna go out on a limb here and say that a 243 probably delivers as much energy on target on deer/antelope size game as much more powerful cartridges do, like the 300 win mag. Yes the 300 has a much more power, but once it passes through the deer, it's not of any use any more. This is just a guess though, as I've never read any credible sources that proved this argument.

That said, I do not believe energy means much when it comes to killing power. For example, a 45 LC pistol only has about 300lbs ME but will cleanly kill any deer out to 50 yds with a 300 gr cast bullet. I believe shot placement and wound diameter are most important.

I've never seen a 243 used on deer, but have seen a 257 Roberts, and it is pretty much the same thing. It works just as well as my 7 Mag, 30-06, 308, and 45-70 inside of 200 yds, and better than my 44 Mag and 30-30.