I agree about using a case/cartridge/headspace gauge. But I don't recommend the Wilson.
A similar gauge from JP Rifles will do the same thing as the Wilson but is cut to match a .223 Wylde chamber and can be used to test both your re-sizing (like the Wilson) as well as testing a fully assembled round (which the Wilson will not necessarily do). If a finished round fits in the JP gauge, it WILL chamber and extract properly in your AR.
http://www.jprifles.com/buy.php?item=JPCG-223