Datums for Dummies

[Slamfire]

Guffey is the center of attention again.

Has anyone looked up the definition of "attention seeking", https://en.wikipedia.org/wiki/Attention_seeking and "narcissist"? https://en.wikipedia.org/wiki/Narcissism Combine those terms and see whose behaviors fits that definition.

 

[stagpanther]

I personally have no idea what you guys are talking about--are you saying someone is coming off a bit high-brow Mr smarty-pants?

 

[Reloadron]

Every time there is a discussion along these lines the thread takes the same direction. The post and responses are pretty much canned and eventually the thread dies a slow and painful death or moderation puts it out of its misery. There is really only so much which can be said on the subject. Eventually, as mentioned, it is a game of semantics or plays on words. As to argumentative there are really no winners and the average person comes away with no knowledge gains. Oh well...

Ron

 

[F. Guffey]

Datums for Dummies

If the question was: What is a datum? A reloader that was curious should have no problem finding one here. Once the definition has been established the next question should be: How is the datum used? Hornady and Sinclair made the jump from comparator to head space gage knowing reloader did not know the difference. There was no reality, reloaders tried to use SAAMI when using the Hornady/Sinclair tool with no way to verify their gage. Reloaders could not understand Hornady/Sinclair built case friendly gages, they used a radius. I like a datum with a sharp edge, I want my datums to cut the case, leave a circle on the shoulder.

Again, there has to be something about the datum that threatens reloaders, I find it impossible to get them to focus.

Elmer Keith was surrounded by snarky smiths, he was doing something they did not understand and not one of them would ask in a civil manner if he would explain. I read through the article about what he was accused of doing. As soon as I read through the part about what he was accused of I knew what he was doing and I knew how he was doing it. None of this stuff drove him to the curb.

And then there is that reoccurring problem: The bolt will not close on a case after sizing; I want to know by 'how much?' If the chamber is too long from the shoulder to the bolt face for the case when measured from the shoulder of the case to the case head I want to know by 'by how much. I know, the standard answer is 'fire form', I had rather form first then fire, that saves me a trip to the range; when I eject a case I eject a once fired case.

Measure from: My favorite cases are cases that have been fired in trashy old chambers, I have gone to firing ranges that allow me to measure the length of cases from the datum/shoulder to the case head before purchasing. There is an almost impossible chance the cases will chamber because of the length of the case from the datum/shoulder to the case head. The up side? There is no way I can miss when sizing a case for a chamber if I know when to stop sizing the case.

F. Guffey

 

[stagpanther]

Hmmm--my opinion is "why is the datum important--what's it used for, and why do you need to know it." I was one of the unenlightened who always assumed head clearance was the same thing as head space. Because there are several mechanically-consequential things that arise out of "know the datum" I don't mind the degree of detail--which sometimes is also referred to as accuracy.

 

[F. Guffey]

Posts: 4,073 Hmmm--my opinion is "why is the datum important--what's it used for, and why do you need to know it." I was one of the unenlightened who always assumed head clearance was the same thing as head space.

That is typical, most reloaders insist the case has head space, SAAMI insist the case does not have head space, I insisted SAAMI did not use the symbol for head space on their case drawings. Is that a problem? No but the person claiming the case has head space takes himself more seriously than he should. And then there are those that take liberties they should not have, those are the ones that call others names. It always starts out by jumping out into the fast lane without enough powder to pass or they miss a gear, and then, they get angry and curse the day they ever started reloading.

When I started on a reloading forum reloaders thought the datum was a line, I insisted the line was a round hole/circle, I gave them three different datum diameters as in .375", .400" and .410", the .375" was no problem and many headed for their hardware store to purchase 3/8" bushings: Problem, the hole in the bushing had a radius around the hole. I then suggested they use the bushing as thought it was a comparator. The designated datum is not necessary if the reloaders measures before and again after.


Many reloaders insisted the shell holder manufacturer had to match the die manufacture even though before the Internet C&H sold dies with the instructions on the bottom of their boxes. C&H claimed the dies in the box had to be used with shell holders with a deck height of .125". The worst shell holders I have ever measured were Pacific, at the time they were identical to RCBS they were integral shell holders meaning the shell holder was part of the ram.

F. Guffey

 

[mehavey]

FWIW to those folk who are new to this game, here's the Bottom Line for use w/ normal/bottleneck cartridges.

Chambers have a headspace dimension: Bolt face to about midway on the chamber shoulder.
Cartridges have a headspace dimension: Case head base to about midway on the case shoulder.

Cartridge headspace dimension ought to be just barely (coupla-thou) smaller than chamber's headspace dimension.

That it all.
Sleep tight.

 

[stagpanther]

For all the fluff and engineer's precision in defining the datum and how it relates to headspace/clearance--I still haven't seen any mention as to why that's REALLY important--and that, IMO, is what would make you a dummy.

because....

 

[Reloadron]

Hmmm--my opinion is "why is the datum important--what's it used for, and why do you need to know it." I was one of the unenlightened who always assumed head clearance was the same thing as head space. Because there are several mechanically-consequential things that arise out of "know the datum" I don't mind the degree of detail--which sometimes is also referred to as accuracy.

First I have to agree with mehavey's explanation. Why is the datum or understanding it important? In my humble opinion the datum is not important and nor is understanding it. While it serves as a point of reference for a measurement it is something the average person hand loading ammunition really has no practical use for.

I fire a cartridge and resize the case, all I need to know is that following the instructions which came with my resizing die my resized case chambers in my rifle I plan to shoot a reloaded cartridge in. That is all that matters for those looking for minute of deer at a few hundred yards need to know. Now for the bench rest serious target shooter it helps to know and understand a little more, however that same shooter is concerned with uniforming flash holes, finished cartridge concentric alignment and a whole bunch more. The truly curious can refer to the SAAMI Glossary of terms for definitions as they apply to the shooting sports.

Ron

 

[stagpanther]

Just stumbled onto what this thread sprouted from--having read the fire-fight on the vintage savage 110. Count me out--never mind my silly comments.

 

[mehavey]

"For all the fluff and engineer's precision in defining the datum and how it relates to headspace/-clearance--I still haven't seen any mention as to why that's REALLY important..."

1st:
DATUM is (by definition) "Reference point"

2nd:
It takes two points to establish your headspace dimension... be it Bolt face and/or case base as point #1

3rd:
The DATUM -- for headspace purposes -- is usually the 2nd point: midway up/on the shoulder... be it chamber shoulder and/or case shoulder.

The distance between those two points defines your Headspace Dimension.


4. WHY IT MATTERS:
- If your case headspace dimension is longer than your chamber headspace dimension... you can't close the bolt.
- If your case headspace dimension is much shorter than your chamber headspace dimension... you overstretch the case -- leading to early failure.

 

[F. Guffey]

4. WHY IT MATTERS:
- If your case headspace dimension is longer than your chamber headspace dimension... you can't close the bolt.
- If your case headspace dimension is much shorter than your chamber headspace dimension... you overstretch the case -- leading to early failure.

And I always ask: "If the bolt will not close why isn't it important to determine by 'how much'?"

And I suggest there has to be a lot of stuff going on the reloader does not understand. I have a M1917 with a chamber that is .011" longer than a minimum length/full length sized case. To save all the reloaders reading this I will save them time as in finding a pencil and paper. The chamber is .002" longer than a field reject head space gage. If what you claim about the long chamber the case would stretch .016" between the case head and case body.

That is not what happens, if what you say is true the length of the neck would not change, if I fired a minimum length/full length sized cases in my long chambered M1917 the neck would get shorter meaning the firing pin did not drive the case forward; instead the case would get longer from the case head to the shoulder.

The insults I get is the price I pay for disagreeing.

If I did not have datums I could not verify a head space gage, I could not make a head space gage. When I started reloaders were using the Wilson Case gage as a drop in gage. The Wilson case gage is a datum based tool. The datum in the case gage has a radius, not a problem; Wilson understands datums, they start with the datum and then grind the case tool to length.

Again, I have no ideal why reloaders are threaten by the datum.

F. Guffey

 

[hounddawg]

Datum
noun, plural data [dey-tuh, dat-uh, dah-tuh]
1.
a single piece of information, as a fact, statistic, or code; an item of data.

http://www.dictionary.com/browse/datum

I just ran across a whole herd of datums over at my bench when setting up for a load test using some new cases. I checked the Peterson cases for volume and found they average about 1.4 grains of water less than the volume of the Lapuas. A couple of datums right off the bat. This datum told me not to expect the same velocities and SD's using the same powder load, primer, and seating depth in the Petersons that I used in the test last week using the Lapua cases

when setting up the sizing die I used my Hornady headspace comparator with to find a datum using a case that had been fired in that rifle but not sized. I took into account the brass springback but since that is minimal it really does not make enough differance to sweat. I could have also used my PTG Go gage to get that datum had I wanted to be super precise. Then I used the datum to set my sizing die to give a .003 shoulder bump when resizing the cases. Oops that .003 would be a datum also. When I calibrated and zeroed the chargemaster and my beam scale. 2 happy little datums scurried off to the land of data

When I decided to start my load test, I used the datum of 38.2 grains of Varget for a a starting weight. Once I finished loading powder into the cases I needed to set my seating die so out came the rifle and my trusty Hornady OAL gage and the bullet comparator along with a .264 datum insert. I used 5 bullets to get an average of distance to the lands and then I had the datum needed datum to determine where I needed to set my die to get distance of .25 from the ogive to the lands. I adjusted my seating die to just barely get the bullet into the neck and measured it with the Hornady gage and there was another datum. This datum was used to determine how much I needed to adjust the die down to get a .025 distance from the ogive to the lands.

Datums are everywhere in reloading and in everyday life as well

 

[Reloadron]

Datums? Rumor control has it they taste like chicken. There are many ways to prepare them with deep fried with a light flour and beer batter being the most popular. Many forums invite members to share their recopies for datums.

Ron

 

[F. Guffey]

And I always ask: "If the bolt will not close why isn't it important to determine by 'how much'?"

One more time: I took a few bolts to a gun show, I laid them on the table. I had a lot of lookers but no one had ever seen a bolt for the 03/03A3 like my bolts. One of my resource people said he would have to dig through his huge gun library. The problem with identifying the bolt gets real complicated when trying to determine how it is used. I have never found it necessary to use a head space gage on the 03, I measure the length of the chamber in thousandths from the shoulder to the bolt face.

Back to determine bolt closing: All I am looking for is the "BY HOW MUCH". A reloader, a smith does not need a head space gage, they do not need a datum, I believe they get lost when they start talking it to death. In the old days it would not have taken 20 seconds out of a soldiers life to measure the length of the chamber from the shoulder to the bolt face (in thousandths) ever day. And then there was Hatcher, same thing.

F. Guffey

 

[Don Fischer]

1st:
DATUM is (by definition) "Reference point"

2nd:
It takes two points to establish your headspace dimension... be it Bolt face and/or case base as point #1

3rd:
The DATUM -- for headspace purposes -- is usually the 2nd point: midway up/on the shoulder... be it chamber shoulder and/or case shoulder.

The distance between those two points defines your Headspace Dimension.


4. WHY IT MATTERS:
- If your case headspace dimension is longer than your chamber headspace dimension... you can't close the bolt.
- If your case headspace dimension is much shorter than your chamber headspace dimension... you overstretch the case -- leading to early failure.

Why is it necessary to know what that measurement is? Seem's to me that what is necessary is to be able to close the bolt on a cartridge without excessive space and without ramming the case into the shoulder of the chamber. If you can do that, what purpose does it serve to know that measurement? I might add, that my sizing dies get set for the rounds in one rifle only. Two rifles in that cartridge require's for me two set's of dies!

 

[mehavey]

Why is it necessary to know what that measurement is? Seem's to me that what is
necessary is to be able to close the bolt on a cartridge without excessive space

Okaaaayy.... You can close the bolt.

You haven't a clue as to how much excess space you now have.

.......... unless.... (drumroll please)

You know (a) the shoulder "datum" length at which the bolt juuuuust closes....
.and then (b) the shoulder length (pesky datum again) which the resized case now has.

That is all...

 

[Reloadron]

Hatcher's Notebook beginning on page 232 for the linked version has some interesting reading and reference material for the shooters, gunsmiths, ballisticians, hunters and collectors. Julian S. Hatcher shares his views and experimentation results on the subject of headspace. Some of the experiment results are interesting material, especially those involving excessive headspace intentionally created.

Ron

 

[F. Guffey]

Julian S. Hatcher shares his views and experimentation results on the subject of headspace. Some of the experiment results are interesting material, especially those involving excessive headspace intentionally created.

I hear that often; Hatcher said etc..

Reloaders assume they can move the shoulder back when sizing the case, I say it is impossible to move the shoulder back when sizing a case with a die that has case body support; and the very next day after saying it is impossible to move the shoulder back with a die with case body support all the reloaders on all reloading forums claim they bump the shoulder back and I ask, "How do you do that?".

And then they claim they move the shoulder forward when they fire the round; I say the shoulder does not move forward when sized and the shoulder does not move forward when fired, I claim my shoulders do not move.

And then there is the claim the case, powder and bullet takes off for the front of the chamber when the primer is hit with the firing pin; and I ask how would a reloader determine if there was any truth in that story.

Hatcher did not get the results he anticipated, I chambered an 8mm57 round in an 8mm/06 chamber, I pulled the trigger, I ejected an 8mm06 with a very short neck.

F. Guffey

 

[Reloadron]

I hear that often; Hatcher said etc..

Never heard that from me? I pretty clearly stated:

"Julian S. Hatcher shares his views and experimentation results on the subject of headspace. Some of the experiment results are interesting material, especially those involving excessive headspace intentionally created".

You have stated over and over again "Reloaders claim" as though you seem to lump all reloaders into a single lot. That is in my opinion far from the case as to reloaders.

You seem to like going off about the term bumping or moving the shoulder back but never once explained your thoughts on the subject.

One more time: I took a few bolts to a gun show, I laid them on the table. I had a lot of lookers but no one had ever seen a bolt for the 03/03A3 like my bolts. One of my resource people said he would have to dig through his huge gun library. The problem with identifying the bolt gets real complicated when trying to determine how it is used. I have never found it necessary to use a head space gage on the 03, I measure the length of the chamber in thousandths from the shoulder to the bolt face.

One more time you never explained why your bolts were different?

Back to determine bolt closing: All I am looking for is the "BY HOW MUCH". A reloader, a smith does not need a head space gage, they do not need a datum, I believe they get lost when they start talking it to death. In the old days it would not have taken 20 seconds out of a soldiers life to measure the length of the chamber from the shoulder to the bolt face (in thousandths) ever day. And then there was Hatcher, same thing.

Again, you fail to mention how a soldier could make the measurement you describe in 20 seconds. Hatcher was pretty clear in the headspace section about the use of gauges, what would your method be?

Ron