The problem with the .243, .308, .260, and 6.5 Creedmore and all those other cartridges that work so well is they'll never fit in an AR15 receiver. Hence, the development of the .300BLK/Whisper, 6x45mm, 6.5 Grendel, and 6.8 SPC/6.8x43mm.
I am currently having a 6.8x43mm upper built due to its performance (doing what you want to do is well within its capabilities) and the fact that it is becoming the mainstream higher performance AR15 cartridge. WalMart ammo availability being the litmus test of that, 6.8SPC ammo was delivered to WalMarts throughout my state yesterday morning - Federal Fusion, 115gr. I've yet to see any of those other options on a shelf at a WalMart.
I am intrigued by the 6x45mm and the .300, because they work with all standard AR parts except the barrel since the cases for them are just necked up 5.56 brass, whilst the 6.8 and 6.5 require different bolts and magazines since they are based on completely different cartridges. I got turned off on the .300 because its performance advantages (duplicate .30-30 hunting rounds, or subsonic heavy bullets with a suppressor) are just not enough to me to justify the expense of another upper. The 6x45mm's easy adaptation from 5.56 brass is very attractive, as is using common 6mm/.244 bullets, but it will be outperformed by my new 6.8 upper using heavier bullets and a larger case.
I don't say that I'll never have either of them, but for now, the only other AR15 cartridge that is intriguing me enough to tempt me to start buying parts for an upper is the .458 SOCOM. Shoehorning .45-70 performance into an AR15 carbine - that's an intriguing round.
Truthfully, though, between the 6.5 Grendel and the 6.8, we're splitting gnat hairs. Both are excellent. I just don't trust how thinned out the bolt face for the Grendel has to be, so I decided on a 6.8. We'll see how happy I am when I get the box of parts I handed my builder yesterday back as an assembled upper.