I have had five 243's. I currently have two 6.5 Creedmoors. I just purchased a 6.5 Grendel. My first choice is the Creedmoor especially if your going out long range for antelope, or ever step up to elk. I think the Grendel would be ok to 300 yards, but I would limit it. Same with the 243. As far as the Barnes X bullets I have killed a dozen deer with them. I have never had one drop. Most run three times as far as with a traditional soft point. If I had a bull elk at 400 yards facing straight away from me and I really wanted him I would want an X bullet in the chamber. I can reach his heart from any direction. However, a broadside shot is never going to stop an X bullet and if you could measure it I am sure more energy is expended in the hill beyond the animal than in the animal. I don't understand the experience others have had of drop them in their tracks. Never happened for me. If your off a bit your never going to recover the animal. I would not have a kid shoot an X bullet.
I shot a lot of X bullets when they were very new (around 1988) until -- well I still have 40 rounds of 300 mag loaded, 50 rounds of 22-250, 50 rounds of 223, 50 rounds of 270 etc. I think the last time I shot any was -- hmmmm after 2010, but I don't remember exactly. It was back when we could buy multiple doe license. I shot three out of five does with X bullets that year. 223, 22-250, and 308. Ooooops I did have two drop in their tracks, but they were through the old brain. No body shots ever dropped them in their tracks.