[Unclenick]

I checked on my .223 PM. The SAAMI maximum .224 Chamber is 1.5713" from head to shoulder datum. On my copy of the PM, the headspace thimble runs out of thread at about 1.5800". So you run out of room for the .204 with not quite a fifth of a turn left. Now the question is, with so little thread engagement can you really trust the consistency of the reading? I suspect the reason they have two full turns of engagement before measuring a .223 is it is the minimum they felt they could trust to align the thimble straight on the case body. If they didn't think they needed those extra turns, why make people waste time spinning in and out of them for every measurement?

Nonetheless, you can test it for yourself. Paint up a fired .204 case shoulder with Magic Marker, then pop it into the .223 PM and measure. Take it out and see if the headspace thimble has marked the shoulder uniformly, or if it has favored one side. Also record the measurement. Remove the thimble and rotate the case about a quarter turn and measure again. Repeat until you have 16 measurements (3 in each position). Average the result and look to see how much any of the individual measurements differ from the average. If its acceptably consistent, that's a good sign. Now resize the case and repeat the marking and measuring. See if the result is any different. If it marks the resized case differently, you are not getting a true relative measurement. If the consistency of the measurements changes you are not getting a good relative measurement. But if they mark the same and the consistency is the same, you are good to go for measuring shoulder setback by the seating die.