I really think we need to go ahead and change over to 6.5 Grendel. There may be better cartridges which will fit the M16 base but any time you can get 6.5 Swedish Mauser ballistics from an M16 then go for it.
There is actually nothing basically wrong with the .223. It just needs to be used in a gun optimized for performance instead of ease of carry.
The 140 gr spitzer in the 6.5x55 smashes deer better than paper ballistics would indicate. Great BC/sectional density. I wouldn't hesitate to use it for elk with a Partition.
We also developed a superb cartridge in the 70s known as the 6mm SAW. It was abandoned. Although I like shooting the .223 I don’t understand why they just didn’t select the .222 Rem Mag which was developed in the 50s as a military cartridge. Better ballistics and a long (reloader’s dream) neck that would suit longer heavier bullets.