Bruce in Oz and others. Re the .223 conversion and more to the point, ejection. Why wouldn't the simple friction ejection as per the standard No4 work. I did it with a 9mm conversion to a No4 body using a shortened 9mm Sterling magazine (or was it a Sten mag......) The 9mm case base rim diameter is the same (?) as .223 Then,the deeper 7.62mm ejector pushed the spent 9mm case over towards the left inner side of the body where it tipped against the ejector and flipped it out. BUT, it would never tip out the LAST round.
On that basis, surely it'd act the same with a .556 spent case