Ok...but if you had FIVE 5.56 AR15's, one 6.8SPC, one .300AAC BO and One 7.62x51/.308Stick with 5.56 and be happy.
...already, and now wanted to build a more customized AR ( or use a company that does that or gives you the options to do it, say like JP Enterprise does...)
...what caliber would you choose...another what?
Or, something different from the above?
6.5 Grendel ?
Another 5.56, .300AAC BO, 6.8SPC...or .308?
The 7.62x51 /.308, has a pretty good balance between factory ammo cost & versatility...
and power, ranging form "Reduced Recoil" loads to Hunting and competition loads.
However, weight & cost to build, is much more.