Since 223 is a factory round, you can also just measure a factory new round before and after firing to get an idea how much the case is expanding. There are different ways to do this, but something easy is to use another fired case, inverted so that it touches the shoulder of the case you want to measure. So for 223, any 30cal case would work. Make sure the primers on the fired case(s) are flush or below flush, and that the 30cal case mouth is not dented. You could even give it a light chamfer to make sure the mouth is smooth and round. Then stick the 30cal case over the 223 round and measure the total length, and write it down. Then fire the 223 and repeat the measurement (again, make sure the primers on the fired case(s) are flush or below flush).
This "tool" for measuring the position of the shoulder also works to adjust sizing dies to set the shoulder back in a consistent way. I have a 9mm luger case set aside that I use for measuring larger rifles cases.
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