Datum line to base cartridge

[marine one]

Hello to all, I'm new member,glade to be hear!
I have a question to all you reloaders, the other day I was at firing range and measured a factory round ( PMC) a 223 with my RCBS mic gauge and it measured .00 as expected,
after firing said round , I measured case again with RCBS mic gauge, and came up with .02 less??
I can't under stand this, should it be on plus side ?
how can you keep firing same case and not bump size it? Normally when you fire a case several times it expands, you eather trim it and or bump sholder back !!!

Thank you in advance for responding to my request.
MARINE ONE

 

[Walkalong]

It's not unusual for the shoulder to be back after firing, as it has expanded in diameter. When you size it, and squeeze the body back in the shoulder moves forward until it hits the shoulder of the die, which then moves it back as you finish sizing. If set wrong the sizer can actually move the shoulder forward. Factory rounds have a range just like anything else, they won't all check in at 0.00.

Size that case and see where there shoulder is then.

Welcome to THR

 

[marine one]

thank you walkalong, for that info,but I don't think you'v answered my question.
I'm not sizing the case i'm only measuring it.
take a factory round out of box , measure datum line to base , fire the round , measure datum line again and I was .02 less not .02 more as it should be.

 

[higgite]

Walkalong is probably right, but try this. Measure a factory round, then chamber it, do not fire it, eject it and measure it again. Your rifle may have a minimum spec chamber. If you were using a bolt gun, was the bolt stiff to close on the factory round?

Also, unless you have verified the zero on your RCBS gauge with a headspace go gauge, don’t count on zero on the scale to correspond to 1.4636” as RCBS claims. I have 2 of them and one reads zero on a go gauge, as it should, and the other reads +.002.

 

[Walkalong]

but I don't think you'v answered my question.

But I did.

It's not unusual for the shoulder to be back after firing, as it has expanded in diameter.

Back as in "shorter" to the base, datum to base that is.

 

[ants]

Yes, Welcome!

As Walkalong said, 'tis true. Maybe hard to picture it but the answer is correct.
That's why you measure and trim AFTER resizing a case.
Straight out of the chamber is the combination of original case size,
chamber dimensions, expansion from pressure, and brass spring-back.
Sometimes it measures as a lengthened case, sometimes not.
Depends upon all those factors.

 

[marine one]

higgite, I tried that, placing a factory round in chamber, closing the bolt and ejecting the round and measuring it -.00
still the same.

 

[marine one]

Point of intrest - my rifle is a CZ 527 bolt action. CIP spec.

. not SAMMI spec.

 

[243winxb]

Sometimes it measures as a lengthened case, sometimes not

This is what i have seen also.

Walkalong is correct.

Some Nato marked Federal ammo will expand to the chamber better then others. This is when compared to my reload. Do not fire Nato ammo in a 223 chamber. (SAAMI)

For brass to expand fully to the chamber, at least 3 firing of neck sized brass is required.

 

[marine one]

Gentelman I thank you all for helping me, but I'm still confused , maybe It's my rifle ( CZ-527 ) Europe spec. not Saami spec.
But I would like someone to take an RCBS precision mic an measure a factory round, fire same round and measure again,
And see what I'm seeing, That's all.

 

[Demi-human]

The chamber spec of the particular rifle makes no difference. The brass is just returning to it's previous state the best that it can. If the neck was annealed well it will be more able to return to the smaller dimensions. If the body is less soft it may expand, to fill the chamber, and stay there. This may shorten the shoulder to cartridge base measurement, as when you increase the volume of the body it tries to pull brass to do so. It depends on the chamber.

And it depends on the brass. Sometimes a case to case difference will cause a measurement variance. A different companies' brass may or may not react to firing the same way.

And not all rifles will experience the same phenomena, even with the identical lot cartridges.

For brass to expand fully to the chamber, at least 3 firing of neck sized brass is required.

He is correct. And he means full power, ran up the ladder, 'for this gun' loads.

Reread Walkalong's first post, but this time thinking, once a brass case stretches, a sizing die can change the shape of the tube, but it can't reduce the surface area. That is why one needs to trim after many sizings. The surface area goes toward the mouth, lengthening the neck, even as they are still sizing to fit in the chamber. The tube must increase in length to maintain the same surface area.

Welcome aboard! I hope I helped a little.

 

[243winxb]

I have to study this CIP data. https://www.scribd.com/doc/14707404...ions-of-Cartridges-and-Chambers-TDCC-Complete

 

[243winxb]

CIP and SAAMI brass and chamber specifications are the same for the most part.

The biggest being .0004" in one cartridge measurement. (L1)

CIP doesn't have a "datum" that i see? Maybe i missed it?
Chamber are the same. Throat/leade may differ like our 3 or 4 types the USA uses.

I would forget the gauge and use the old method. Back the FL die out so there is .005" between the shell holder and FL die. See if the sized brass chambers. If not screw the die down more. A slight crush fit when closing this bolt is getting almost there.

 

[higgite]

FWIW, SAAMI and CIP chamber dimensions are virtually the same, so I don't think it's an American vs European spec problem. I have read before that CZ rifles tend to have chambers sized closer to minimum than most other mfrs. I know that’s true for my CZ pistol compared to other brands.

 

[ants]

I would help you, marine one, but I don't have an RCBS gauge like yours.
I use the rifle chamber to set the die for full length resize, rather than a gauge.
Not that I think the gauge is wrong, it's just that I started reloading more than 40 years ago
when we didn't have all the tools and instruments available today,
so I developed other practices and habits. Never did get any of the
RCBS precision micrometer headspace gauges, but I think they're pretty cool.
Maybe I'll get some extra money some day and get one.
Currently I reload about 2 dozen different rifle cartridges,
not sure which Precision Mike would be the one to start.

 

[Walkalong]

It doesn't have to be an RCBS Precision Mic, it can be whatever you use to measure where the shoulder is. Measure it factory new, measure it fired, measure it sized. We will still see what the shoulder did. Just use your preferred method.

This is one way.

 

[Mizar]

CIP doesn't have a "datum" that i see? Maybe i missed it?

It has a datum point, it's just measured differently - look at the "S" dimension, Junction cone.

 

[Blue68f100]

Hello to all, I'm new member,glade to be hear!
I have a question to all you reloaders, the other day I was at firing range and measured a factory round ( PMC) a 223 with my RCBS mic gauge and it measured .00 as expected,
after firing said round , I measured case again with RCBS mic gauge, and came up with .02 less??
I can't under stand this, should it be on plus side ?
how can you keep firing same case and not bump size it? Normally when you fire a case several times it expands, you eather trim it and or bump sholder back !!!

Thank you in advance for responding to my request.
MARINE ONE

I've used the Precision Mic on my 7mm Mag mainly for setting up for 2 different guns. Since your getting 2 different readings on the same brass you need to practice so your consistent. It's like any micrometer it takes a feel. With the PM you can over tighten it. Make sure your brass is clean as any dirt in the wrong place will impact the reading.

 

[243winxb]

It has a datum point, it's just measured differently - look at the "S" dimension, Junction cone.

47.05mm= 1.852"
47.12mm = 1.855" Looks like case head to where the rifing starts. To long to be our datun measurement.

Or did i miss something again? This is making my head hurt.

 

[Mizar]

Knowing two things - distance from the cone point to cartridge base (breechface) and the cone angle, one can determine where the shoulder section of the cone exactly is. It's no different than the SAAMI datum point - a distance between a plane, cutting a circle with predetermined diameter from the shoulder cone and the cartridge base (breechface).

 

[higgite]

The CIP “S” dimension can be calculated from cone angle and shoulder location, but good luck measuring it.

 

[Mizar]

"Datum point - Any reference point of known or assumed coordinates from which calculation or measurements may be taken."
Dictionary of Military and Associated Terms. US Department of Defense 2005.

One is not calculating S dimension, one is calculating other dimensions using S dimension as a basic.
Datum line in SAAMI chamber drawings is not a headspace, it is exactly what it says - a datum (line, point, plane, whatever it must be), a basic dimension. It is used to set the shoulder cone to the desired position. One is not measuring headspace dimension to the datum line, but to the cone. As much as the datum line is important, so is the shoulder cone angle - change the angle and that gauge will no longer measure correctly. Or, one can grind away the cone bellow the datum line diameter and that gauge will still measure correctly.