New Brass size question

[mac37]

I posted this in the 6mm forum but it is more of a reloading question.
New, XC chamber measurement question.
All measured with the Tubb comparator gauge.
JGS go gauge 2.063
3x fired brass 2.0645/2.065
Now the new Norma brass is 2.058/2.059
So, max case stretch is .007? Does that sound right?

Cases resize fine with the Tubb die and rifle shoots great.
Any info is much appreciated.

 

[dkhunt14]

I don't know what you are really asking. Do you mean the new brass is .007 shorter to datum? That is a normal measurement from new to fired brass. Once they arefired they should form close to the fired dimension. Matt

 

[mac37]

Yes, wondering if a .007 size increase from new unfired to fireformed is normal.

 

[noload]

Yes its normal for New brass to blow out .007" longer. Some new brass is closer to your chamber size and does not blow that far. If you are questioning your chamber size? check it with Head space gage.

 

[mac37]

Chamber closes on a go gauge and will not close on a no-go gauge.
Using the tape method and confirmed by fired brass it's .002" past the go gauge.
I was just concerned that the Norma brass was too short. My 6br and 6 creedmoor's new brass is closer to the size of the go gauge, within .002". Thanks for the info.

 

[Brians356]

Chamber closes on a go gauge and will not close on a no-go gauge.
Using the tape method and confirmed by fired brass it's .002" past the go gauge.
I was just concerned that the Norma brass was too short. My 6br and 6 creedmoor's new brass is closer to the size of the go gauge, within .002". Thanks for the info.

I have measured new factory ammo that is .010" shorter at the datum than after firing in a SAAMI chamber. And that is well within spec. It is not atypical to find SAAMI Maximum Chamber minus Minimum Cartridge (i.e. worst case) to be a large as .014".

 

[riflewoman]

The go gauge is the minimum chamber. The no go is .004 inches more than that.

The cartridge maximum is the go gauge dimension. Manufacturers make their cases under the maximum because they need to allow for manufacturing process variables. In the cases at hand that is .004 under the minimum chamber/maximum case.

Your situation illustrates why case sizing affects case life.

 

[fguffey]

So, max case stretch is .007? Does that sound right?

You are assuming the brass stretched. If my cases stretch I want to know from where, I want to know to where. I am thinking it is a bad thing when the shoulder on my cases moves when fired.


I formed and then fired wildcat cases that shortened .045”; the shoulder on my formed and then fired cases did not move.


F. Guffey

 

[fguffey]

I have measured new factory ammo that is .010" shorter at the datum than after firing in a SAAMI chamber. And that is well within spec. It is not atypical to find SAAMI Maximum Chamber minus Minimum Cartridge (i.e. worst case) to be a large as .014".

Brian356, I check the length of the chamber from the shoulder to the bolt face first. I have an Eddystone M1917 that has .011” added between the shoulder of the chamber to the bolt face. When sizing cases for that chamber I adjust the sizing die off the shell holder .014” then size 280 Remington cases to 30/06.

The cases after sizing are the same length as a filed reject chamber. The chamber is .002” longer than a field reject length chamber. Adjusting the die .014” off the shell holder give me the magic .002” ‘BUMP?” clearance. Again, I am the fan of cutting down on all that case travel, if that is what it does.

In the perfect world the chamber would be go-gage length and the case would be minimum length/full length size. In the perfect world I should have .005” clearance.

F. Guffey

 

[ireload2]

Yes, wondering if a .007 size increase from new unfired to fireformed is normal.

You can find out what is normal several different ways.
1. First you can check the standard for the brass minimum length and the chamber maximum length. That will give you theoretical worst case stretch presuming that you start out with a max chamber.

2. Or if you know what your chamber is you can find out from the case specification what the minimum case is likely to be.

3. You can measure each lot of cases and compare to your chamber

4. You can measure each lot of cases and perform statistical analysis of the case variation.
You would have to calculate the standard deviation of the sample and calculate plus and minus 3 standard deviations.
This would tell you how far out to the left the outliers are on short side and how far the outliers are on the long side. This would tell you how much variability Norma had in that run of cases. If you don't know how to do this, it is not difficult and I can do it for you if you have the measurements of all the cases.

I have done this for a different cartridge - the 7.65 Mauser.
PPU is a little bit more uniform than is Norma and PPU statistically about .003 longer for both ammo and new brass.

 

[Brians356]

Brian356, I check the length of the chamber from the shoulder to the bolt face first. I have an Eddystone M1917 that has .011” added between the shoulder of the chamber to the bolt face. When sizing cases for that chamber I adjust the sizing die off the shell holder .014” then size 280 Remington cases to 30/06.

The cases after sizing are the same length as a filed reject chamber. The chamber is .002” longer than a field reject length chamber. Adjusting the die .014” off the shell holder give me the magic .002” ‘BUMP?” clearance. Again, I am the fan of cutting down on all that case travel, if that is what it does.

In the perfect world the chamber would be go-gage length and the case would be minimum length/full length size. In the perfect world I should have .005” clearance.

F. Guffey

I'm sure there is a point you are making, but I cannot be sure what it is. WRT the OP's query, I stand by my assertion that up to .014" shoulder movement forward, starting with an unfired case, would likely be within SAAMI specs for "Min Case" in a "Max Chamber". That's worst case and unlikely, but .007" shoulder movement is not only no indication something is wrong, it is to be expected (again, in a SAAMI situation - with a custom bespoke chamber and a non-SAAMI cartridge like 6 XC, the owner may well expect the builder to have tightened things up considerably.)

 

[fguffey]

I'm sure there is a point you are making, but I cannot be sure what it is. WRT the OP's query, I stand by my assertion that up to .014" shoulder movement forward, starting with an unfired case, would be within SAAMI specs for "Min Case" in a "Max Chamber".

Brian356, I find reloaders on reloading forums have a lot of trouble keeping up. Most blame me; and that is OK by me. If I fire a case and it stretches .014" I am of the opinion the case was rendered scrap. I do not believe SAAMI specs mean 'anything between minimum and maximum is within specification'. My opinion; a reloader should be able to determine the length of the chamber from the shoulder of the chamber to the bolt face. But what is a reloader to do when everything has head space and every tool is a head space tool.

I am the fan of cutting down on all that case travel.

F. Guffey

 

[Brians356]

Brian356, I find reloaders on reloading forums have a lot of trouble keeping up. Most blame me; and that is OK by me. If I fire a case and it stretches .014" I am of the opinion the case was rendered scrap. I do not believe SAAMI specs mean 'anything between minimum and maximum is within specification'. My opinion; a reloader should be able to determine the length of the chamber from the shoulder of the chamber to the bolt face. But what is a reloader to do when everything has head space and every tool is a head space tool.

I am the fan of cutting down on all that case travel.

F. Guffey

Fair enough. I hope I never implied someone should expect .014" of shoulder migration, but it is a fact that SAAMI specs account for that much as the worst case. But if you go buy several brands of common factory ammo, and fire them in several off-the-shelf factory rifles in some SAAMI chambering, you will almost certainly find the shoulders haved moved forward after firing on the order of ~.007" (+-.002"). To be surprised and even alarmed about that is to be uninformed about industry standards and manufacturing practices. As I stated, that does not apply to a custom bespoke chamber, which may well be expected to closely fit the unfired brass to be used in it.