Unexpected Results using RCBS Precision Mic

[jimbires]

I'm pretty sure what's causing the brass to get shorter is , the brass has expanded to fit the diameter of the chamber causing it to get shorter . another way to think of it is , as the brass got wider it got shorter . another couple of cycles and the brass will stretch to fit the length of the chamber . this is why we should not cut brass trim length on new brass , after a fire or two the brass could be short .

 

[RD57]

I use the Precision Mics all the time, and what you're experiencing is not unusual. The Mic is great for a reference but is not to be used/can't be use as a measuring tool like a ruler. I have a Sako 75 .223 Rem that the Prec Mic indicates a -.006 for fire formed cases (2 - 5 times fired). The chamber is within SAAMI spec. When I full length size my brass with a carefully honed down #10 shell holder it is -.005 and I don't feel any resistance with the bolt closing at all. Factory unfired .223 ammo I've used usually indicates about -.003 headspace with the Mic. Using the same Prec Mic my VTAC II 5.56 chamber indicates +.002 headspace after firing the same LC brass several times. As mentioned before don't crank down on it, do clean the bottom where the brass head sits and do keep your brass clean. I've seen errors also from extractors raising the area of the brass head creating a un-square head.

 

[Patriot007]

The RCBS Precision Mic is a great tool. But I think you might want to try the Hornady head space comparator gauge..
I "think" it might work better for your process. One thing I noticed about the RCBS precision mic is. The base of the case should be perfectly squared or it will give you a false reading. It's like using a hornady anvil base on a caliper.
It will give you a false reading. JM2C

 

[243winxb]

as the brass got wider it got shorter .

Strange but true. This is why we fl size before trimming.

 

[nicholasjohn]

This is rather puzzling. I haven't used this tool, so I can't comment. I have used the Stoney Point comparator, and have never had a problem with it.

 

[Beluebow]

It's like using a hornady anvil base on a caliper.
It will give you a false reading. JM2C

How so?

 

[DrSteve]

I'm very grateful for all the thoughtful comments. I got out my Hornady Comparator and using a light touch on the RCBS Precision Mic, I ran the numbers again, and the results were much more consistent. On my reloads (I only have three unfired ones left so the sample size is small). The Hornady comparator said the datum length went from 1.621 unfired to 1.623 fired. The RCBS Mic said the length went from -.002 to -.003, which is counter-intuitive, but may be measurement error. On the Hornady American Whitetail factory loads (I measured about 10 of these all from the same lot, and the readings were consistent) the Hornady Comparator said the datum length stayed the same at 1.623. The RCBS Mic said the length went from -.002 to -.001", so that's going in the right direction, I guess. The directions said to add the fired measurement to the SAAMI minimum (1.630") to get the chamber headspace. In this case, my chamber is apparently .001" smaller than SAAMI minimum. I've ordered a Forster Go Gauge to see if it will chamber. If the Go Gauge won't go, should I send the rifle back to Ruger or just work around it? Thanks everybody.

 

[misterc01]

This is rather puzzling. I haven't used this tool, so I can't comment. I have used the Stoney Point comparator, and have never had a problem with it.

FYI - Stoney Point is now the Hornady OAL gauge.

 

[misterc01]

I use the Precision Mics all the time, and what you're experiencing is not unusual. The Mic is great for a reference but is not to be used/can't be use as a measuring tool like a ruler. I have a Sako 75 .223 Rem that the Prec Mic indicates a -.006 for fire formed cases (2 - 5 times fired). The chamber is within SAAMI spec. When I full length size my brass with a carefully honed down #10 shell holder it is -.005 and I don't feel any resistance with the bolt closing at all. Factory unfired .223 ammo I've used usually indicates about -.003 headspace with the Mic. Using the same Prec Mic my VTAC II 5.56 chamber indicates +.002 headspace after firing the same LC brass several times. As mentioned before don't crank down on it, do clean the bottom where the brass head sits and do keep your brass clean. I've seen errors also from extractors raising the area of the brass head creating a un-square head.

Intersting - RCBS instructions seem to indicate it an actual measuring device, and Honday instrucitons indicate it is a relative mesuring device. I use both just to be sure. AND I also use the Sinclair comparator as well. So I personal preference being, well, personal, I consider the Precsipn Mic the more accurate device.

 

[Patriot007]

How so?

I'm Going to copy and paste my reply from a post a while back on this subject and give you a direct link to the thread.
I find that the Hornady anvil Base has a tendency to obscure my readings. That being said, this is why I say this, and I own 4 of these. In new brass you want see this as much, but in cases with 10 to 30 firings on them. "Lapua 6.5CM, SRP, annealed after every shot." It's very obvious, The base is no longer squared or it has snags from the extractor and ejector. When you place the bullet case on the perfectly flat anvil it will show a different reading verses not using it. That's in a bolt rifle,,,. Now if you really want to see some thing that will have you scratching your head. Use it on some 223/5.56 brass fired out of a AR-15 or 308 out of a AR-10, after one firing.
I loved the concept of the anvil, but, I'm doing just great with out them. Mixed readings when your OCD weighs heavy on the mind, like chasing lands. LOL ,,,,,
Here's the link to the tread on this forum. enjoy.

Hornady OAL Gauge is Driving me Crazy(er)!

I spent a couple hours today trying to get consistent CBTO readings on my Ruger Hawkeye 270 and some Nosler ABLR bullets. The readings are all over the place. I've had more consistent results with some other bullets. I try to be consistent, firm but not too firm, give it a couple taps. I...

www.longrangehunting.com

 

[Beluebow]

I'm Going to copy and paste my reply from a post a while back on this subject and give you a direct link to the thread.
I find that the Hornady anvil Base has a tendency to obscure my readings. That being said, this is why I say this, and I own 4 of these. In new brass you want see this as much, but in cases with 10 to 30 firings on them. "Lapua 6.5CM, SRP, annealed after every shot." It's very obvious, The base is no longer squared or it has snags from the extractor and ejector. When you place the bullet case on the perfectly flat anvil it will show a different reading verses not using it. That's in a bolt rifle,,,. Now if you really want to see some thing that will have you scratching your head. Use it on some 223/5.56 brass fired out of a AR-15 or 308 out of a AR-10, after one firing.
I loved the concept of the anvil, but, I'm doing just great with out them. Mixed readings when your OCD weighs heavy on the mind, like chasing lands. LOL ,,,,,
Here's the link to the tread on this forum. enjoy.

Hornady OAL Gauge is Driving me Crazy(er)!

I spent a couple hours today trying to get consistent CBTO readings on my Ruger Hawkeye 270 and some Nosler ABLR bullets. The readings are all over the place. I've had more consistent results with some other bullets. I try to be consistent, firm but not too firm, give it a couple taps. I...

www.longrangehunting.com

Funny how a tool works for one guy and yet it's worthless to another...like they say there is more than one way to skin a cat and not all of them are wrong....I've used the anvil for years and measured thousands upon thousands of bullets and cases with zero issues.

 

[Patriot007]

Intersting - RCBS instructions seem to indicate it an actual measuring device, and Honday instrucitons indicate it is a relative mesuring device. I use both just to be sure. AND I also use the Sinclair comparator as well. So I personal preference being, well, personal, I consider the Precsipn Mic the more accurate device.

I will agree with you provided that the case head is squared. That's what I have found to cause the issues with cases fired out of my AR-15 & AR-10, and in my bolt rifles with mutable firings on the cases. The case has to sit perfectly squared, "FLAT" to get an accurate reading. It's the little things like this that can drive a man insane. OCD doesn't help the level of madness involved with the tedious aspects of it. LOL

 

[misterc01]

I will agree with you provided that the case head is squared. That's what I have found to cause the issues with cases fired out of my AR-15 & AR-10, and in my bolt rifles with mutable firings on the cases. The case has to sit perfectly squared, "FLAT" to get an accurate reading. It's the little things like this that can drive a man insane. OCD doesn't help the level of madness involved with the tedious aspects of it. LOL

It is not OCD! To be correct, it is CDO so it is alphabetical order!

 

[Patriot007]

Funny how a tool works for one guy and yet it's worthless to another...like they say there is more than one way to skin a cat and not all of them are wrong....I've used the anvil for years and measured thousands upon thousands of bullets and cases with zero issues.

I know, wright. That's what I love about this Hobby, and the people on this forum. We can SKIN that CAT in a many of different flavors. LOL