Opinated, just FYI that an AR15 can have other calibers other than 5.56/223 i.e. 6.8SPC, 6.5G, 300BLK, 458Socom, and 50Beowulf.
You are not limited to the 5.56/223 on an AR15, which is the beauty of that rifle. Swap a different upper and you can have muliple calibers depending on your intended use.
Btw 5.56 is not "weak". Try getting shot with M855 green-tip 5.56 ammo wearing a ballistic vest and see if you'll survive...
If you want a .308, then sure go with AR10/Scar/FAL etc. Just saying that the 5.56/223 is not as weak as many portray them to be, and you are also not limited to that ammo in an AR15 platform.