I have killed a lot of elk in my 55 years of hunting them and in about 1/2 of that time I also guided hunters so I have seen a LOT more killed then those I have killed myself I have seen them killed with rifles as small as 243s and as large as a 577 Snider, and 62 caliber flintlock. Bullet construction is far more important then BC, SD, velocity, and all the marketers hype they try to force feed you.
I have killed them with 7X57 Mauser, 7MM Rem Mag and 7MM Weatherby mag. If using good bullets they break the big bones just fine. So does a 270 Winchester with 150grain partitions and the old Remington Core-Lokts and the 160 grain Partitions too. In fact the old Remington bullets were superb. In my years of guiding the 2 calibers that gave the worst bullet performance overall were the 7 Mags and the 300 Mags and 100% of those failures were because of using bullets that were to fragile for an animal that large. And yet if loaded with proper bullets they all worked like magic. I took a man out many years ago who came from new Jersey to hunt in the Selway and he shot a bull that ran off a ways. He has a 300 Weatherby Mag and loaded it with 165 grain bullets (I think they were Speer Hot Cores) We caught up to the elk about 500 yards from the place he hit it (still took us about an hour to find it) and I shot it 3 times with my 44 mag. He made a comment how a handgun was worthless if his 300 Weatherby didn't drop it, but was shocked to see all 3 of my round exited his bull and his bullet went only about 14 inches deep and didn't make it to the other lung. Mine were all hits in the mid or rear body as it ran away, and all 3 came out the front of the chest. After that he started to listen when I told him what bullets' to bring. He hunted with me 4 times in the next years and every time after that he showed up with 220 grain bullets loaded. Never has a problem after that.
That is one of several stories I can relay about such lack of performance and shallow penetration's, but in case of a gal shooting a 243 with Barnes X bullets she did fine. Not all exits, but all went clear through and were on the skin on the off side.
A 7MM mag (or a 7-08 or 7x57) is fine for elk if you load "elk bullets" in it. My old favorites were the 160 and the 175 grain Partitions, but today you can pick from others that are fine too. Stay away from those that break up and you will be fine even if you need to take a quartering shot.
