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Old August 8, 2012, 10:42 AM   #1
tpcollins
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Anyone have both .204 and .223 RCBS Precision Mics?

I sent an email to RCBS asking if the bases for .204 Ruger and .223 Remington were the same and whether I could just order the headspace gauge for .204 and use it with my .223 body. They replied that the bodies were different.

I'd like to know if they're truly different or not? If someone has both, they ought to be able to tell for sure, thanks.
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Old August 8, 2012, 03:32 PM   #2
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If you look at page 16 of the RCBS replacement parts catalog (not yet updated to include the .204), you will see that while you can buy the ogive nut and the bullet guage (freebore tool) separately, the body and case headspace nuts are sold only as sets. I believe that's because they have to be checked individually with a headspace GO gauge to see that the graduations line up close to zero, then they are kept paired. This is because that's an absolute measurement. The land nut is just used to seat the bullet relative to wherever it happens to measure the freebore tool to be. They sell those as interchangeable, but I'll bet any two randomly purchased examples might not hit exactly the same number on the same dummy cartridge. If they differ by 0.005", so what? As long as you use the same one to measure their freebore gauge and to adjust your seater die, you are good to go.
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Old August 8, 2012, 05:04 PM   #3
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Thanks Nick, that makes sense. I was all over their site this morning and couldn't find the link you listed, thanks again.
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Old August 8, 2012, 05:11 PM   #4
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I think Unclenick is absolutely right. But, is there any reason you can't use the .223 Rem headspace nut as a comparator/bump gauge for .204 Ruger?
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Old August 8, 2012, 07:13 PM   #5
tpcollins
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I was thinking the same even though the .204 is longer but the specs indicate the .204 has a 30 degree shoulder angle whereas the .223 is 23 degrees - bummer . . .
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Old August 8, 2012, 11:27 PM   #6
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Yeah, different shoulder angle, but same datum diameter .330" and that's what the Prec Mic indexes on. I still think you could use your .223 PM as a comparator/bump gauge for .204, but I was wrong once before.
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Old August 9, 2012, 09:32 AM   #7
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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.
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Last edited by Unclenick; August 9, 2012 at 09:42 AM.
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Old August 9, 2012, 10:53 AM   #8
higgite
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Good catch, Unclenick. I hadn't thought about that. My .223 Rem PM runs out of threads very close to where yours does. With it run out to the max SAAMI headspace for a .204 Ruger cartridge (1.5713"), it still has thread engaged, less than 1/4 turn, same as yours, plus the fit is very sloppy. At .204 cartridge max h/s (1.5645"), it's better but still a sloppier thread fit than when it's in the .223 h/s range. I agree with you (I know you're thrilled to hear that. ), it would be iffy at best to use the .223 PM for .204 measurements. Don't you just hate it when factory CS techs know what they're talking about?

Thanks for the exchange. Hope the OP got something out of it.
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