Is there a difference? Brownells states that Clymer's gauges are the same for both 5.56 NATO and .223 Rem, however, .223 gauges are $19.99 and 5.56 gauges are $34.99.
The datum line on the shoulder is the exact same between the two and Head space gauges don't measure the biggest difference in .223rem/5.56mm which is the throat of the barrel