I have not always been a huge fan of the other calibers for AR15's, mostly because of the lack of ammo availability.
That said, of all of the other popular AR15 calibers outside of .223/5.56, I have always liked the 6.5 Grendel the best. It's quite a bit bigger and heavier than the 5.56.
Here you can see how it compares to the two most common NATO rifle rounds, (from left to right) the 7.62, 6.5, and the 5.56, courtesy of www.65grendel.com.
It was designed by Alexander Arms to fill the gap between these two rounds, giving the AR15 more long range capability than the 5.56, with a light recoil.
Recently, my brother built up a 6.5 Grendel AR15 for his wife and kids to use while hunting Elk near their home in Idaho. The AR15 is just a better platform for them to use, with the accuracy combined with the adjustable stock, etc...
This season his wife drew a Bull Elk tag close to where they live. They got out to an area where they had seen Elk previously and she was able to get a beautiful shot from about 50 yards away. This bull did not take another step and dropped after being hit with a handloaded 125gr Nosler Partition.
Although the 6.5 Grendel may be a bit on the small side for Elk and requires good judgment and shot placement for a clean kill, you can clearly see that it did it's job here as good as anything.
I recommend taking a close look at this cartridge if you are needing something with some better range in an AR15 platform. It's not just a good hunting round.
That said, I would highly recommend that you reload for this round as ammo is very tough to get a hold of.
Here is a close up of the rifle in the building process, which started out as a 5.56, but was later converted by swapping out the 5.56 barrel with a 20" fluted Sabre Defense 6.5 Grendel Barrel. The camo job was done with a two part epoxy paint with hardener very similar if not identical to Duracoat made by Sherwin-Williams called Polane T.