While some of this is true, the important factor is head space in the barrel and the bolt. An improperly head spaced barrel/bolt for 5.56 is just as dangerous as a .223 with 5.56mm in it.
Bottom line is that any AR owner knows (or should) that they need to spend time measuring their parts if they assemble it themselves, check the head space, and the leade. Most completed guns are already head spaced in the factory and are set for 5.56.
Service parts are an issue that needs to be considered in an AR type rifle. A matched set barrel/bolt is significantly more expensive than either of the parts individually. Not that setting the headspace is that difficlut, and the gauge block is $15 or $20. Obviously, too long can be corrected (shaved to meet the spec), too short is dangerous and hard to determine even with the gauge.
If you buy a top of the line Bushmaster or Colt from the factory set up the way you want it in the 5.56 it will be perfectly on the money. If you spend half as much and get a knock off AR you have no idea about headspace or where the parts came from. Bottom line is step up and spend $1,200 - $1,900 for a top grade weapon.
Our M-4 Bushmaster has matchgrade barrel and trigger and is stamped 5.56 on the barrel and the receiver. .223 in this weapon is not very accurate however when we use Lake City or Federal 109’s in 55 grain FMJ or 62 grain tracers we get a 2 inch group at 200 yards. Again this is what you get when you buy from the factory set up the way you want it.
If you want an off the shelf weapon get an Ak.