The smallest cartrdge I have ever killed elk with was the 270WSM but it was only 50 yards away and a bang flop neck shot with 140 grain Failsafe which penciled through the neck. It was a smallish cow too. I had about 3 seconds to get off the shot. My brother shot a larger cow with same cartridge and we had to track it for several miles even though it was terminally wounded in the vitals. The next year (this year) we traded those rifles in for 30-06's. And we will be shooting 180 grain Nosler Partitions, or possibly 168 TXS. I agree that bullet diameter is meaningfull. Personally, I now put my minium at 30-06. But I have absolutely nothing against those that select smaller, less capable cartridges as thats a personal choice we are all entitled to. I feel that it best to be equipped with something that will perform with less than ideal shot positioning with bone breaking capacity. Ideally, my personal favorite is the 338 WM for elk. Every one I have shot with it went nowhere fast. The shock that cartridge provides is devastating even compared to the 30-06. Right now my philosophy is that I only have time to get real competent with one rifle, so I've elected the 30-06 for both deer and elk and anything else I will hunt. I believe in developing intimacy with your rifle no matter what cartridge you select. I will say that in my elk hunting life I have had very few shots like on TV where they wait and wait for just the right moment for that easy broadside shot right behind the shoulder. My experience is that the window of opportunity has always closed very fast and one needs to be prepared for fast decision making and knowing where to place shots at different angles......and having a gun and shooter that can pull it off without a hitch. The only shots I have ever passed on were either too far away (rare) or not enough time in the clear, or lastly elk going straight away with only butt showing