Precision Mic Question

[TAC308]

I purchased the RCBS Precision Mic and have a few questions:

1. I took (5) fired cases and measured them. They range from 0 to +.25. Do I take an average of them all to determine how far out of spec my chamber is?

2. If I measure an unfired Lapua case (which I plan to use for reloading) it comes in under 0. I'm guessing this is normal since the case has not been fired and has not stretched to match the chamber. Is this correct?

I'm sure I'll have more questions once you guys start to respond...

Thanks in advance.

 

[WARDOG]

Re: Precision Mic Question

What are you using to determine your "zero"? (Chamber guage, RCBS die set at "0", lowest number measured, or SAAMI case length?)
Are you talking about measuring the OAL?

 

[TAC308]

Re: Precision Mic Question

The instructions tell you to measure fired cases to determine headspace, correct?

 

[Fuzzball]

Re: Precision Mic Question

"The instructions tell you to measure fired cases to determine headspace, correct?"

No, and the gage numbers are largely arbitrary. Headspace is in the chamber, what we measure with the gage is only the head-to-shoulder length of the case and that's by no means the same thing as headspace, nor need it be.

What the actual headspace in a rifle's chamber is means nothing to a knowledgeable reloader. All we need do is use the gage to match the (max) resized shoulder length to what the (max) fired length is and all will be well.

Forget averaging anything in reloading, that only obscures what's happening. You will quickly find that neither fired nor resized shoulder case length is precisely repeatable. Cases in the same box do differ in both fired expansion and sizing, and the springback after each.

 

[427Cobra]

Re: Precision Mic Question

Take it back and buy the Hornady tool

 

[EddieNFL]

Re: Precision Mic Question

Are you sure of your measurement?

 

[Fuzzball]

Re: Precision Mic Question

"Take (the RCBS Case Mic) back and buy the Hornady tool"

Since both of those tools accomplish exactly the same thing I don't understand the suggestion. ??

 

[WARDOG]

Re: Precision Mic Question

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: TAC308</div><div class="ubbcode-body"> I took (5) fired cases and measured them. They range from <span style="font-weight: bold"><span style="color: #CC0000">0</span></span> to +.25 ... </div></div>
We would like to help but this question <span style="font-style: italic">still</span> needs to be answered.
You wrote ".25" DO you mean .025". The way you have it written, ".25" is 1/4 of an inch. You <span style="font-style: italic">must</span> mean .025".
Decimal first tenths, hundreds, thousands.
So what is the "zero".

 

[427Cobra]

Re: Precision Mic Question

The Hornady/Stoney Point is cheaper and must be easier to use because I never see threads from people who don't understand how to use it.

 

[TAC308]

Re: Precision Mic Question

Leave it up to the douche bags in the forum to run off at the mouth and offer no help.

For those that don't mind helping me out - what I meant to say was the gauge reads anywhere from 0 to 2.5 lines (or .0025" per the RCBS manual).

I'm asking what this all means. Does it mean that I simply check fired cases to make sure the don't exceed this number and if they do I have to full length re-size them?

 

[RH4540]

Re: Precision Mic Question

Even though you didn't say what caliber you are checking, I don't think it makes any difference.

New brass SHOULD be at zero. I've seen readings as high as 5 out of some rifles, without any issues. If you are at 2-2.5, you have a pretty tight chamber.

When you resize your brass, the resized brass needs to be a lower reading than your fired brass, otherwise, you may not be able to get the bolt to close.

I hope this helps!!

 

[WARDOG]

Re: Precision Mic Question

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: TAC308</div><div class="ubbcode-body">...what I meant to say was the gauge reads anywhere from 0 to 2.5 lines (or .0025" per the RCBS manual)... </div></div>
Yeah, if you are not used to reading micrometers it takes a bit to catch on.
.0025 sounds like much less of a problem with the first measurement you gave.
So your brass is forming to your chamber at .0025" (two and a half thousands of an inch, or read 25/ten thousands of an inch) over <span style="font-style: italic">standard</span> brass size. That is not bad. Your chamber could be another .001" longer but the brass case is shrinking slightly after the pressure is released.
<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: TAC308</div><div class="ubbcode-body">...I'm asking what this all means. Does it mean that I simply check fired cases to make sure the don't exceed this number and if they do I have to full length re-size them? </div></div>
YES! EXACTLY! You should get a few firings without having to full length size them. When te bolt starts to get hard to close, that is the sign that you have to FL (Full-Length Size the body). When you <span style="font-style: italic">do</span> have to FL size them, resize the shoulder back another .001", or .0015 reading to allow it to fit to your chamber again. When you FL size a piece of brass (Unprimed, Not charged), seat the bullet the way you have been doing it, and make sure your bolt is closing before you FL size the rest of your brass shot the same number of times. If the bolt <span style="font-style: italic">does not</span> close, then you have a dimension wrong, your micrometer has been moved out of calibration, and you should try to move the shoulder back another .001".
I hope this answer finally helped you.

<span style="font-weight: bold"><span style="color: #993300">WARDOG</span></span>

 

[btm_54]

Re: Precision Mic Question

The Hornady tool works and gets repeatable results.

 

[Ratbert]

Re: Precision Mic Question

The problem I have with the precision mics is that they only measure on a single arbitrary spot on the shoulder and they don't measure seating depth on the ogieve. Consequently the measurements, while (probably) consistent, are for the most part arbitrary and cannot be related to any measurements taken with any other device or even by measurements taken using a different bullet. The '0' marked on the engravings has absolutely no relation to anything, you will need to use a headspace gauge and your own chamber in order to determine what "0" means in your rifle.

The way I would do it (if I hadn't tossed the things in the bin and gotten the hornady and redding instant indicator) would be:

Get a GO gauge for your chosen chamber. Make sure the bolt WILL close on it (otherwise you've got bigger problems.) Assuming yes, put it in the Mic and measure it. Right that number down. Then take a piece of scotch tape stick it on the bottom of the headspace gauge and use a razor blade to trim it so you have a nice little circle of tape on the bottom. Put this in your gun and try to close the bolt. If it closes easily, take it out and measure again with the headspace gauge. It should be 0.001 to 0.002" longer. Right that down and add a 2nd layer of tape to the gauge and repeat. If it just BARELY closes, but requires a fair bit of force then take it out and measure again, THIS is your new zero. You want to size your brass so that they're 0.002-0.004 under this number(depending on your preferences for reliability vs absolute accuracy and your equipment's tolerances when sizing). If it won't close at all (which may be the case if your tape is on the thicker side) then take that measurement and split the difference between it and the last measurement where the bolt closed easily to find your zero.

 

[TAC308]

Re: Precision Mic Question

To the last 3 gentlemen who provided a usable response - thank you.

 

[bignada]

Re: Precision Mic Question

Every RCBS Precision Mic is different. They are not calibrated to one standard. Doesn't mean they aren't useful, but you need to understand that fact.

To measure your chamber dimension, measure brass before and after you fire it. Might also measure some virgin brass to see what the manufacturers deliver. Look also at loaded ammunition. These can easily be up to .050" below the -0- index on your Mic tool.

The -0- index does not mean SAAMI minimum or Go-Gauge minimum, but unless you are hunter of dangerous game, a tight or short oal chamber is better. The less expansion the brass makes to fill your chamber, the longer it will last. If you shoot a wildcat ctg, your chamber can be as short as you desire, because presumably no one else will make your ammunition. If you are cool with your brass mfrs typical oal dimension on virgin brass, you can cut your chamber to be .0025" longer, and then resize by .001 or .0005" and basically preserve brass integrity until you blow out the primer pocket. Should never see a separation.

Using a Precision Mic or Wilson Case Gauge to investigate rimmed magnum chamber is very smart practice. You want headspace on the shoulder, not rim for a magnum. If chamber is long, your brass won't last.

Taylor your brass oal to chamber and work with brass that never expands more than .0025 to begin with, and only .001 or .0005 thereafter on each firing, and you will have brass life that is excellent. Not to mention, probably the best accuracy you'll ever experience. Chamber and ammunition, particularly bullet seating are the primary accuracy components, not "blueprinted actions or even custom barrels.

Precision Mic is a very fast tool to use, easy to read, and doesn't require owning a dial caliper. Still they are expensive.
If you own one, learn to use it and setup your sizing dies to produce only minimum shoulder bump back. Redding Type S FL bushing sizer is a great choice to get most out of a precision rifle w/short oal chamber...

Good luck!

 

[EWP]

Re: Precision Mic Question

My RCBS .223 Mic 0's out perfectly on factory new Hornady .223 brass(loaded ammo or component brass) since Hornady holds their brass to a better standard than most, it may be worth a call to Hornady to see just what standard they hold their brass to(SAAMI min or max) since every lot I have is sized the same.

 

[Ratbert]

Re: Precision Mic Question

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: EWP</div><div class="ubbcode-body">My RCBS .223 Mic 0's out perfectly on factory new Hornady .223 brass(loaded ammo or component brass) since Hornady holds their brass to a better standard than most, it may be worth a call to Hornady to see just what standard they hold their brass to(SAAMI min or max) since every lot I have is sized the same. </div></div>

Or you could just get a go gauge of known length, put it in the mic and find out what you get.

 

[Innovative]

Re: Precision Mic Question

I don't recommend trying to learn how to operate the RCBS Precision Mic in a forum. This is a good tool when used according to RCBS instructions. However, you'll need to buy another one when loading each different caliber. (RCBS can email you another set of instructions.)

Hornady's tool is a bit easier to use. However, it's more expensive, because you'll need a collection of expensive bushings to reload different calibers.

Check out the <span style="text-decoration: underline">Digital Headspace Gauge</span>. It works on <span style="font-weight: bold">ALL</span> different calibers, and it doesn't require any bushings or special adapters.
Check this out for simple operation:

1.) Insert one of your fired cases, and set the v-block anywhere on the shoulder.

2.) Press the ZERO button to calibrate the gauge to your chamber.

3.) Insert one of your handloads, and the gauge will display the clearance "at the shoulder" that YOUR handloads have in YOUR particular chamber.

4.) Use this info to adjust the height of your sizing die.

 

[woodspider]

Re: Precision Mic Question

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: 427Cobra</div><div class="ubbcode-body">The Hornady/Stoney Point is cheaper and must be easier to use because I never see threads from people who don't understand how to use it. </div></div>
+1 THIS well said

 

[Ratbert]

Re: Precision Mic Question

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: woodspider</div><div class="ubbcode-body"><div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: 427Cobra</div><div class="ubbcode-body">The Hornady/Stoney Point is cheaper and must be easier to use because I never see threads from people who don't understand how to use it. </div></div>
+1 THIS well said</div></div>

The Stoney Point doesn't address headspace at all, which is really the crux of the OP's question.

 

[Fuzzball]

Re: Precision Mic Question

"The Stoney Point doesn't address headspace at all, which is really the crux of the OP's question."

No case gage does, or can, address "headspace". That term relates ONLY to the chamber; case gages relate ONLY to the case; new, fired or unfired.

Done properly, we can precisely measure a fired case to an arbitrary point and then adjust our FL sizing so the shoulders match a chamber pretty closely. THAT is the whole point of using such gages!

 

[Fuzzball]

Re: Precision Mic Question

"Leave it up to the douche bags in the forum to run off at the mouth and offer no help."

It's difficult to properly respond to a poorly worded question and it appears to be difficult to accept a proper answer when people don't even understand what the hell they're trying to accomplish.