RCBS PRecission Mic

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Yosemite Steve

New member
I finally got it!

The only thing I don't understand is why it does not include a shoulder piece to measure chamber headspace. The instructions say to fire a factory round and measure it. To me that is not an acceptable way to insure a precise measurement so I made my own shoulder attachment that replaces the ogive piece. It works perfectly.

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Now, finally! I can measure the case head to datum in thousandths of an inch reliably. BUt that's not all! I can also measure where the ogive meets the lands AND chamber length.

The federal factory un-fired round is 2.047". My Savage 30-06 chamber is 2.051". My Enfield 30-06 chamber is 2.056". Thanks, Richard for the great deal on the tool! $35 well spent!
 

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Great buy! Just started long distance bolt action so I was wondering how important of a piece of equipment it was.

Sent from my SM-G950U using Tapatalk
 
Transition!
Now you are using your eyes and your mind to figure out your own answers.
Those mics are recommended by some very knowledgeable people.I'm sure they are a good tool.
I've been too cheap to go that route!
If I decide to go that way,I'll ask you questions.
 
I've been down the RCBS Precision Mic path and I haven't used any of the 6 calibers in which I invested for the last 10 years or so. The major problem I had was reproducibility of the initial measurement. Do you gently spin the unit to a stop? Do you turn it gently instead? Both of these procedures reveal just a little more turn pressure might be available as you think you're there and start to unscrew it. I finally gave up and switched to a Stoney Point Comparator and their Cartridge Headspace Gauge. Both include required caliber tools, negating the need for individual caliber investment. I find the reproducibility of measurements much easier to accomplish.

If anyone is really enthusiastic abut the Mics, you can buy the ones I have.
 
The only thing I don't understand is why it does not include a shoulder piece to measure chamber headspace.
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When determining the length of the chamber from the datum/shoulder to the bolt face the reloader is instructed to fire a case and then measure the length of the case from the shoulder/datum to the case head. It gets confusing when reloaders claim the case does not fit the chamber until it is shot 5 times and then there are those snap back, jump back and spring back reloaders that make fire forming impossible.

I want to know the length of the chamber from the datum/shoulder to the bolt face before firing.

F. Guffey
 
Don't trust the headspace nut to zero at "minimum ANSI level" as advertised until you check it with a known standard, such as a go gauge. Two of the 3 I've had didn't zero on go gauges. Until you verify its accuracy, I would only use it as a comparator. ymmv
 
I want to know the length of the chamber from the datum/shoulder to the bolt face before firing.
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why do you think you need to know the exact measurement if that measurement is within SAMMI specifications as measured by gages

I use go and no go gages that were manufactured to SAMMI specifications, to make sure my rifle does not have a case failure on the first firing after replacing a barrel or bolt face. After the first firing any comparator and a fired case will give all the info needed to adjust my die

@ Yosemite perhaps I am missing something here but what does that tool do that a Hornady comparator not do ?

https://www.hornady.com/reloading/p...gauges/headspace-comparator-anvil-base-kit#!/
 
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Hounddawg, It was a tool that a friend had that is useful. He only wanted $35 for it.

I have already found it useful in that I can examine what is happening to my brass when it is fired. Some of the light loads that I have shot did not expand the shoulder as far as others and now I can measure that. When I put together a batch of brass for testing accuracy I want it to be as consistent as I can make it. I can now sort my brass further by shoulder length.

This tool with my modification can tell me how much longer or shorter that my chamber is than the brass I am working with. I can now size my brass to fit how I want with less guess work. I do not own a Hornady comparitor so I can't say which is better. I am satisfied with what this tool is telling me. That is all that matters to me.
 
Hounddawg, It was a tool that a friend had that is useful. He only wanted $35 for it.

I have already found it useful in that I can examine what is happening to my brass when it is fired. Some of the light loads that I have shot did not expand the shoulder as far as others and now I can measure that. When I put together a batch of brass for testing accuracy I want it to be as consistent as I can make it. I can now sort my brass further by shoulder length.

This tool with my modification can tell me how much longer or shorter that my chamber is than the brass I am working with. I can now size my brass to fit how I want with less guess work. I do not own a Hornady comparitor so I can't say which is better. I am satisfied with what this tool is telling me. That is all that matters to me.
Click to expand...

maybe I worded my question wrong, I have a habit of that and often owe an apology. I meant no disrespect to the RCBS tool, RCBS makes fine tools. I was just wondering if I needed to think about acquiring one. I already own the Hornady tools but if this does something different or better I could possibly fit one or two into my budget sometime this spring.

BTW you seem to have come a long way since the bolt face question. This stuff is not rocket surgery but there is so much to learn I find I learn new things almost on a daily basis
 
"...shoulder piece to measure chamber headspace..." Cartridges do not have headspace. Particularly chamber headspace. Headspace is a rifle manufacturing tolerance only.
"...Savage 30-06 chamber is 2.051"..." Nope. SAAMI spec minimum is 2.502" +2 thou.
 
No one else believes they have a use for 'the datum', I do not need another one but if I did not have one I would drill a straight hole that measured .375" in diameter or I would use a take off barrel or I would cut a shot-out barrel off and use the chamber end for measuring the length of the case from the shoulder/datum to the end of the protruding case head.

Same with the chamber gage, if I did not have one I would make it, my chamber gages do not look like Wilson case gages because I do not measure with my thumb nail, I measure case head protrusion with a gage. After all these years Wilson has started making dial indicator holders for use with case gages.

F. Guffey
 
Seem's I just read about this somewhere a day or so ago. I've had one for about 25 yrs now. Haven't a clue what use it is! Just got it out again and a 25-05 MTY and loaded round out. Doesn't make one bit of sense to we what I'm supposed to be seeing! You can measure to the datum line, so what? I can neck size with a FL die till the bolt won't close anymore then work the case through the FL die adjusting it several times till it fit's. Whatever that measurement might be, who care's and why? I even re-read the directions again and haven't a clue what is going on. I find the tool useless, if it does give some kind of measurement, I find it of no use at all!

This tool is the only one I've ever had like it. I found it so confusing and useless I finely put it away and never even considered another. At the time I got it I had already been reoading for about 25yrs. How did I get through 25 yrs without the thing that now that I have it, haven't a cue how it works? For myself it is the most useless reloading too I have ever seen.
 
Steve,

Good job on squeezing an improvement out of the existing tool for just the cost of a case. I love a dual-use freebee. Wish I'd thought of it. Nothing like fresh eyes on a problem to bring about innovation.

It is worth mentioning that it is best to use a fired case for this with .30-06 and .270 Win and .35 Whelen because looking at the SAAMI drawing you will see the cartridge and chamber have slightly different shoulder angles owing to the original headspace determinate was from before the shoulder datum standard was adopted and was the shoulder and body intercept. For that reason, they wanted the outside edge of the shoulder to make contact first, even if pressure and maybe just rapid enough chambering were going to press them together. Interestingly, there is no universal agreement about the amount of difference as I've noticed the SAAMI drawings and Garand drawings don't agree in this regard. But I digress.

The value of that measurement will be that you can monitor headspace growth accurately. Any relative measurement should do that, but I find all cases don't spring back the same amount, so yours is a better way to look out for a growth trend.

You will still need a good quality headspace GO gauge if you want to calibrate the Precision Mic itself. The three I have are all off about 0.002" at zero (the GO gauge length), two minus and one plus.
 
I've been using the RCBS Precision Mic for years . I first wanted to know my chamber lengh , from bolt face to the datum . I ordered a go Gage , for a 308 caliber it's 1.630 . By using only the bolt body on a Rem. 700 I could add a piece of shim stock to bolt face. My chamber is 1.632 , by installing the go Gage in the mic and adding .002 I know my measurement of zero case space or case headspace. The adjustable bullet that comes with the mic is useless. If you can strip your bolt an check by feel for headspace an ogive zero setting , you can bump your brass to what ever works best or jump or jam your bullets. The mic is the only tool I use for full sizing my 308 reloads. Also use the Redding Competition Seating die & shellholders set of 5 . Works great for me. Hope I helped.

Chris
 
Where do you come up with 2.502"?
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The case length is 2.294", the chamber is longer. Most reloaders use the 'trim to length', I want to cover my chamber with the case, I have no problem keeping the cases trimmed to 2.494".

F. Guffey
 
Yosemite Steve said:
T. O'Heir, pardon my miswording. I meant chamber space from datum to boltface... bolt closed. Where do you come up with 2.502"?
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Steve, you're addressing headspace dimensions and he quoted chamber length to where the leade starts, but he got it wrong. The dimension that he referred to is 2.502" +15 thou, not +2 thou.
 
Really a pretty slick improvement and thanks for sharing that.

Higgite mentions:
Don't trust the headspace nut to zero at "minimum ANSI level" as advertised until you check it with a known standard, such as a go gauge. Two of the 3 I've had didn't zero on go gauges. Until you verify its accuracy, I would only use it as a comparator. ymmv
Click to expand...

That would be quite true. Overall I have found the RCBS Precision Mics to be within +/- 0.001". I compared them to Forrester and Clymer headspace gauges which I had the opportunity to check and before retiring I got the mechanical lab guys at the plant to check my headspace gauges on a Zeiss CMM system so I know them to be good.

Here is what higgite is getting at:
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I believe the above is a .308 Winchester RCBS Precision Mic. The idea behind checking the RCBS gauge is the same. Measure a known and the gauge should read correctly.

Headspace gauge inserted into the RCBS Mic.
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Yosemite Steve, thanks again for sharing that, pretty slick.

Ron
 
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