Measuring Case Body Length Correctly

[RBT321]

I'm still new to hand-loading but I'm trying to get things right...

When I measure the body length for my 6.5CM the slight curve at the transition between the body and the shoulder means that measuring the body length can vary by up to 40 thou depending on where exactly your datum is along that tiny radius. I'm trying to figure out which datum point is the correct one to use.

The three choices seem to be:

1. The beginning of the curve, where the body starts to bend inwards.
2. The end of the curve, where the shoulder flattens
3. The mid-way point between the two, where the angle of the body and the angle of the shoulder would meet

An example with an un-fired Hornady 140gr ELD Match round:

1. 1.4716in / 37.38mm
2. 1.4956in / 37.99mm
3. 1.4836in / 37.69mm (calculated average)

I'm guessing that the correct answer should really be the third option, the mid-way point where the two angles would meet, but I'm not sure.


And the same question also applies to the shoulder <> neck interface too

Thanks!

 

[rwj]

What is compelling you to measure these case features? Most employ a functional inspection using a case gauge or the barrel chamber to determine if the case fits. Resizing is typically limited to that required to permit free chambering.

 

[HandgunHTR]

I will ask the question that I believe @rwj is asking a different way?

What are you trying to achieve? Are you trying to measure how much you are bumping your shoulder? If so, then get a decent set of comparator inserts, such as this one (https://shortactioncustoms.com/product/modular-headspace-comparator-kit/) and use the correct insert. Measure a fired case. Zero your calipers on that case. Resize it and measure the case again. Record how much bump you got. Adjust die until you are getting 0.002-0.003" bump.

 

[chromatica]

The datum point is actually midway up the shoulder. The best instruments have a collar of the exact diameter of the shoulder at the datum line. I like the RCBS Precision mic. You drop your case into the bottom tube and screw the top down til it stops. Then you take the micrometer readings. The Precision Mic is calibrated to read zero on SAAMI min (GO gauge). First, do a sanity check with your GO and NO-GO gauges in the Precision mic. Then measure the cases coming out of your sizing die to see exactly how long they are and whether you are getting the right amount of setback.

The Precision mic gives very repeatable results - not like the rickety systems that clamp onto your calipers.

 

[DShortt]

I've always looked at datum lines with a critical eye.

Yes, there needs to be an established measurement point.

Does that take into account that the shoulder degree may be off in either the chamber or sizing die?

No, it does not.

So. I don't get 'wrapped around the axle' over things that, by nature and design, will never be perfect.

 

[Jason D Hostetter]

If you build a custom rifle, it's a great idea to get your gunsmith to make you a bump gauge with a piece of the barrel stub.

 

[RBT321]

Sorry guys, I'm not talking about the ogive or headspace datums. I'm trying to measure the body length from the head to the point where it transitions into the shoulder. In this SAMMI image its the line that measures 1.4901in (37.849mm):



I got curious about this because only a few of my cases (both un-fired factory and fired re-sized cases) actually meet the SAMMI official figures. Most have a body length (and neck length) that comes up short of even the minimum SAMMI figure.

Regardless of whether I measure the start of the transition or the end, neither seem to be long enough to meet the spec.

That piqued my curiosity so I've been doing some more measuring and now I'm just wondering which part of that tiny radius curve I should be measuring to, the beginning, end or middle. I figure it's the middle, but wanted a more knowledgable person to weigh-in.

What is compelling you to measure these case features? Most employ a functional inspection using a case gauge or the barrel chamber to determine if the case fits. Resizing is typically limited to that required to permit free chambering.

My "inner OCD engineer" is showing!

For hand-loading I am indeed mostly relying on my ogive comparator and headspace comparator, with a visual check for any defects.

As an aside; my spreadsheet of measurements did catch one case that had an unusually wide shoulder, and when I tried, it wouldn't chamber. So unexpectedly these extra measurements actually did something useful!

I've read a variety of great books on the subject, and being new to this, I simply want to understand for myself exactly what is happening to the cases when they are fired. So on my first few batches, I'm measuring everything I can, before and after firing, in order to build a detailed picture in my own head.

I don't expect to keep measuring everything long-term, but comparing the numbers for different types of brass has been informative.

 

[rwj]

My "inner OCD engineer" is showing!

For hand-loading I am indeed mostly relying on my ogive comparator and headspace comparator, with a visual check for any defects.

As an aside; my spreadsheet of measurements did catch one case that had an unusually wide shoulder, and when I tried, it wouldn't chamber. So unexpectedly these extra measurements actually did something useful!

I've read a variety of great books on the subject, and being new to this, I simply want to understand for myself exactly what is happening to the cases when they are fired. So on my first few batches, I'm measuring everything I can, before and after firing, in order to build a detailed picture in my own head.

I don't expect to keep measuring everything long-term, but comparing the numbers for different types of brass has been informative.

Best wishes to you on the measuring journey! I’m thinking access to a CMM would be handy and provide the best accuracy. Remember, cases will fire form to the shape created by the chamber wall regardless of their factory/original configuration.

 

[SeabeeKen]

Sounds like over thinking to me. Brass forms to the chamber when fired. I just want my shoulder bump perfect and straight necks

 

[Sandstorm]

Are you using Lapua Brass.?

 

[VA_XTC_Shooter]

You could get a small surface plate and a height gage. Mark the brass with dykem. Set the height gage to 1.4901" and scribe a line on the brass at that height. Then compare with multiple pieces to see if the scribe line is in same location at the junction.

 

[JEFFPPC]

I know one thing. The OP won't be the guy you need to beat at a match.

 

[BKA]

See the little asterisk and triangle symbols in the 1.4901 dimension line...they mean something.
Read the fine print.
* = DIMENSIONS ARE TO INTERSECTION OF LINES.
Triangle = REFERENCE DIMENSION

 

[Jerry Wick]

I know one thing. The OP won't be the guy you need to beat at a match.

That's really uncalled for.

A litte extra measuring doesn't make him a bad shooter and he admits he's new to reloading. There's nothing wrong with trying to learn.

 

[SKWERLZ]

Sorry guys, I'm not talking about the ogive or headspace datums. I'm trying to measure the body length from the head to the point where it transitions into the shoulder. In this SAMMI image its the line that measures 1.4901in (37.849mm):

View attachment 1676428

I got curious about this because only a few of my cases (both un-fired factory and fired re-sized cases) actually meet the SAMMI official figures. Most have a body length (and neck length) that comes up short of even the minimum SAMMI figure.

Regardless of whether I measure the start of the transition or the end, neither seem to be long enough to meet the spec.

That piqued my curiosity so I've been doing some more measuring and now I'm just wondering which part of that tiny radius curve I should be measuring to, the beginning, end or middle. I figure it's the middle, but wanted a more knowledgable person to weigh-in.



My "inner OCD engineer" is showing!

For hand-loading I am indeed mostly relying on my ogive comparator and headspace comparator, with a visual check for any defects.

As an aside; my spreadsheet of measurements did catch one case that had an unusually wide shoulder, and when I tried, it wouldn't chamber. So unexpectedly these extra measurements actually did something useful!

I've read a variety of great books on the subject, and being new to this, I simply want to understand for myself exactly what is happening to the cases when they are fired. So on my first few batches, I'm measuring everything I can, before and after firing, in order to build a detailed picture in my own head.

I don't expect to keep measuring everything long-term, but comparing the numbers for different types of brass has been informative.

Why?
I think you’re overthinking it.

 

[K22]

It's a hobby so you can measure anything you want if that is your thing. I just do not need to know the measurement in question that the OP is asking about.

Now let's talk practical. I am no expert, but this is my understanding, and this has worked for me to produce optimum sized cases.

The datum line is a reference point to measure a fire case (primer removed) to aid it setting up your F/L die to obtain optimum sizing. Once cases are fire formed to a specific rifle chamber, they become conformed to the rifle chamber and thus serve as somewhat consistent measurement basis for setting up your F/L die.

You can optimize the process by using virgin cases from the same lot and of course, dedicating them to a specific rifle, rotating their use so all cases in that batch receive the same number of firings and sizing's. While I have never done it, I understand proper annealing also increases the consistency.

I think that is all there is to it from a practical viewpoint. It's an efficient and measurement-based system that works quite well for me. In my opinion, it is one of the most important steps in the reloading process both from a functionality and precision standpoint.

 

[JFrank]

DIMENSIONS ARE TO INTERSECTION OF LINES.

The intersection of lines would certainly be the correct point of measure.

 

[Doom]

Understad that the cartridge case dimensions are Maximum. Chamber dimensions are minimum.

 

[RBT321]

The intersection of lines would certainly be the correct point of measure.

Thank you! That's exactly the answer I was looking for!!


As for the rest, this is just satisfying a bit of curiosity on my part. I've worked for F1 and I design rockets and satellites for a living currently, so I live in a world where empirical testing is critical. I've always enjoyed understanding things better and I enjoy seeing the results of real-world testing based on what I read from informed sources. In this case Jeff Siewert's Ammunition, Demystified was a great read and I wanted to measure for myself, some of the effects that the brass experiences when it is fired. You wouldn't be the first to call me a bit odd, but I just find the dynamics to be fascinating.

I've got a range of different brass. Reloading cases from Lapua & Starline, together with fired factory ammo from Norma, Hornady, Winchester & Federal. It has been interesting seeing the tolerances each use - and yes, I can already see in the numbers that Lapua tolerances are noticeably tighter than the others. I haven't tried any Alpha brass yet, but I intend to get some sooner or later and I expect to see their numbers give Lapua a run for their money.

Some of the cases have been fired once or twice already and next I want to measure again after annealing them. But I'll need to build me an annealer first

I know this archive of measurements is excessive and unnecessary for making good reloading ammo, but it's scratching an itch that I've got right now. I'm sure that when I fill the hole of understanding to my satisfaction, I'll stop measuring every last thing. All just another part of the hobby for me!

 

[JEFFPPC]

That's really uncalled for.

A litte extra measuring doesn't make him a bad shooter and he admits he's new to reloading. There's nothing wrong with trying to learn.

He's not off to a good start. Lots of numbers seem to be rage these days.