AR-15s
Look into the other brands available on the market. Colt's are good but limited to few options. The aftermarket tends to give you a much wider choice and you can talk to some of them just like you would talk to an engine builder, and tell them what you are looking to do. They can recommend the right barrel, caliber combinations.
Personally Mine are chambered in .223 Wilde.
This chamber safely shoots any 5.56 or .223 ammunition out there.
Not everyone is aware of the pressure differences in the 2 calibers.
Physically they are the same but the loadings are different between commercial loaded .223 and the Military 5.56 loading.
You can safely shoot .223 in a 5.56 chamber, but 5.56 in a .223 chamber has a much higher pressure. Not recommended.
.223 vs 5.56 NATO vs .223 Wylde: Will Your Gun Blow?