i dont think you can hunt big game in this state with a caliber of .223. thats the point Wyoming will now let you. Colorado isnt ahead in that. Yes, you have allways been able to hunt varmints with a .223. But, not big game.
From the Colorado DoW
"Rifles using center fire cartridges of .24 caliber or larger, having expanding bullets of at least seventy (70) grains in weight, except for elk and moose where the minimum bullet weight is eighty-five (85) grains, and with a rated impact energy one hundred (100) yards from the muzzle of at least one thousand (1000) foot pounds as determined by the manufacturer's rating providing that any semiautomatic rifle used shall not hold more than six (6) rounds in the magazine and chamber combined. "

