I'm a firm believer that 500ft.lbs. is the minimum theoretical energy that one should have upon impact-although I've seen a 9mm pistol (usually around 350ft.lbs. at the muzzle) take a deer at 45yrds-knocked it in the mud with one shot. Another thing that I'm also a firm believer in is bullet diameter and weight, with a .243, the biggest factor of energy is the speed, not the bullet weight, with a .45colt, the bullet does the work, because obviously it's not going that fast... kind of like getting whipped really hard with a hickory sprig-stings like hell but goes away after a bit-lightweight moving very fast, but if you barely get thumped with a 2x4, it's gonna leave a mark-heavy not moving too fast...With a .243, I'd certainly like at least 750ft.lbs. upon impact, if not 1000, whereas with a .45colt, 500ft.lbs. I KNOW will get'er done. That's the whole reason there aren't as many small bore hunting pistol cartridges around, compare the .30carbine to the .45acp, the .30carbine is considerably longer, and much faster than the .45acp, but I guarantee if you wanted a defensive pistol, you'd want a .45acp because of the big ol'bullets that carry a load of stopping power.

One other thing about comparing rifle cartridges to pistol cartridges, as far as ranges go, rifles usually carry way too much energy for their own good-what I mean by this is a .30-06 or a .300win mag can easily kill a deer at 1000yrds, but there are about 6 guys (non-military) in the world that could make that shot under hunting conditions...it's more powerful than you could ever shoot it accurately, rifle ranges are typically determined by accuracy, NOT energy...Pistol cartridges, on the other hand, get limited two fold, past a certain range, say 100yrds with a .44mag, it's getting VERY hard to hit the boiler room even with a glass, so that's a limiting factor, additionally, most pistol cartridges are getting pretty weak by this point, the .44mag is dipping to about 650-700ft.lbs. (in my loadings) at this point, so it's energy is also limiting it to that range...most .45colts I've shot lose accuracy AND energy at about 100yrds as well, .357's are about 25-45yrds shorter.