Yep, all factory ammo. My favorite two is the Sig 220 OTM and Hornady 208 Black

I have 2 6.5 Grendel's and 1 6.8 SPC. I've broke 2 Grendel bolts trying to get more speed. They stay in the safe anymore while the 6.8 SPC goes hunting. Both lose too much steam to hunt past 3-400 yards. But the SPC has more steam getting there
  If I want to shoot long range I have a 6 ARC or several bigger cartridges. The Grendel just doesn't have it. Not sure why the love besides riding the coat tails of the CM

It's interesting to shoot 6.8SPC and 6.5Gr next to each other.

The shape of 6.5Gr brass makes it feed like a truck going over a clunky railroad track. The 6.8 feeds smoothly.

I never noticed it until I shot them side by side.

both are 17-19 clicks elevation to get a 100yrd zero to 400yrds.

both CLANG 400yrd steel equally loud,  much louder than .223

I like them both a lot better than .223

If I had to pick one, it would probably be 6.8SPC

If you hunt.

Absolutely.

Hunting further than a football field? Either of the 6.x rounds, 6.5 Grendel uppers can be more cheaply build out than 6.8 SPC in my experience, but otherwise some folks prefer the feel of one over the other but they're both great out past a quarter-mile.

Most other purposes I'm a fan of .458 SOCOM. Sure the ammo gets pricey but it hits like a dumptruck and (like 300BLK) favors very short barrels so the platform ends up very light and more rear-biased without going all the way to a bullpup.

And where .458 runs out of steam the 6.x options handily take over. And both benefit from the same lower changes so they're near-ideal sibling-uppers to have IMHO.

So while 5.56/.223 is a great all-around, it really comes down to "short or long?" if you're trying to "do better" IMHO.

In addition to my 5.56s, I have a couple of 6.8s (from ARP) which are very capable rifles. 6.8x43 makes an AR about the best it can be within the limits of the original mag well. Grendel has a very spotty reputation on bolt/extractor issues and that’s an absolute NO GO for me.

For simple fun at the range, a 9mm 635 clone is probably my favorite. With duty grade hollow points, it’s quite adequate for defensive use around the house in the event I need to repel boarders.

If I added something besides 300blk and 5.56 it would be a 6.5 vs 6.8 debate. I tend to prefer the 6.8 due to better/more reliable mags (Barrett and Magpul) and a better/meatier bolt. Plus I almost never shoot past 300 so 6.8 wins there too.

However, that said, there's also 300 HAMR, which is basically 300blk if it wasn't hamstrung by being a dual-purpose cartridge.

Lastly, 224valkyrie is interesting if you want to shoot far and have an extra .224 caliber dedicated suppressor laying around, since none of the others will fit through the hole.

That said, for now, I'm sticking with 300blk and 9mm as my other than 556 AR calibers.

6.8x51mm, basically a necked down .308, but with a steel case head that will withstand 80,000 psi operating pressures. 135 grain Sierra Match King bullet.

This round (not the high-pressure version) can only fit in AR-10 frames. Standard AR-10's cannot handle the high-pressure ammo.

IMO nothing beats 5.56mm. Nothing.

Ammo availability and price.
Logistics in a national emergency.
Premium ammo can fill the hunting gap.
The AR-15 was designed for 5.56mm, everything else is being shoehorned into a platform that is not ideal for anything bigger.
Magazine reliability, though this has improved significantly over time.

I own;
6mmWOA
6.8SPC
6.5 Grendel
7.62x39mm AR-15's.

IME the oddball calibers are more reliable when using 15/17 round magazines. Do not buy any "straight" 20 round magazines, you need curvature to help with feeding.

AR-15's in 7.62x39mm are twice as accurate as AK's in the same caliber.

I prefer light bullets in 6.5 Grendel and 6.8SPC. The heaviest bullets I use in 6.5 G is Lapua's 108's, I use Sierra 115 Match Kings as my heaviest bullet in 6.8SPC. 90, 95 and 100 grain bullets in both calibers achieve velocities that are typical of real high power rifles in both calibers. 115's in 6.68 and 123's in 6.5 are pretty anemic when it comes to SAFE velocities.

5.56mm loaded with 75/77 grain match bullets is hard to beat for long range accuracy and reliability. Being cheaper and more readily available is a plus. Speer Gold Dots or Nosler hunting bullets can serve as a minimum deer load with good shot placement.

IMO the 6.x calibers the best for the assault rifle concept.  Soviets went too big, Americans went too small.

my vote is for .223 Wylde

Keep in mind that is not a caliber itself, it is a chamber that splits the slight differences between .223Rem and 5.56 NATO.

I forgot to add, 277 Wolverine is a very interesting option, but such limited choices for people that don't reload kind of relegate it to niche status.