Datums for Dummies

[Jim Watson]

You will note I said head SPACE, not headspace. Unclenick says the boffins at SAAMI call that head clearance. OK. I think the thread has gotten dictionary proud and away from Reloading needs. And I mean it is not important like magazine vs clip.

[higgite]

Everybody who's anybody knows that ammo doesn't have headspace. Hrrrumph.

Hrrrumph Indeed...

Twenty-two posts now, on somethinga that everybody know doesn't exist.

<smirk>
---

Yes, hrrrumph indeed indeed….

I didn’t say everybody, I said everybody who’s anybody. We know who we are. That would of course include Mr. Guffey, myself and two others who shall remain nameless. And a childhood acquaintance who got his head stuck in an old paint can pretending to be a Knight of the Round Table, but that’s drifting OT.

[F. Guffey]

Edit - just to make it more confusing I discovered belted cartridges have a datum point on the shoulder of the SAAMI drawings. That is the diameter that you would measure from to determine shoulder setback

I have said reloaders are infatuated with head space and the word 'bump'. I have described the length of the chamber as; from the datum/shoulder to the bolt face. Because the case does not have head space I off set the length of the chamber with the length of the case. I determine the length of the case by measuring from the datum/shoulder to the case head.

SHOULDER SETBACK: I measure before and again after, the case does not have head space so I measure the length of the case from the datum/shoulder to the case head. When finished I determined clearance by subtracting the length of the case from the shoulder to the case head from the length of the chamber when measured from the datum/shoulder to the bolt face.

I know how easy it is to confuse reloaders. Years ago L. WILLIS started making a dial indicator stand and called it a digital head space gage; reloaders heaped praise on him and then there was the 3 point contact, same thing. At best the tool would qualify as a comparator and had there been one reloader that had a clue how the datum is used he could have made added his creation to his dial indicator stand. When finished he could call it a head space gage.

I call my gages 'gages', some are used to measure the length of the case from the datum/shoulder to the case head. And for levity? I claimed I took a picture of my gages, the picture weighed 800 pounds.

F. Guffey

[stagpanther]

And a childhood acquaintance who got his head stuck in an old paint can pretending to be a Knight of the Round Table, but that’s drifting OT.

made my day--thanks for that.

[Don Fischer]

I've read through this thread a couple time's and hove absolutely no idea what this is about or how it would effect handloading. Your case either fit's the chamber properly or it doesn't. If it is to short you'll have a headspace problem if the case is resized to spec's and the chamber simply a bit to long. If the case is to long, the bolt won't close on the cartridge. If I have this measurement to the data line on the case fine, for what ever good it may be. But say I have it, how do you measure the chamber to find where it is in there without buying an extremely expensive tool of some kind to measure it.

It would seem to me that all I'm really interested in is hot the case fit's in the chamber and the datum line don't mean squat to me except for a discussion that seem's to be un-necessary for the average reloader. I don't care one bit about the datum line, I only case if my case fit's my chamber. So, where is this thread supposed to lead?

[F. Guffey]

Edit - just to make it more confusing I discovered belted cartridges have a datum point on the shoulder of the SAAMI drawings.

'You discovered?' I have suggested reloader purchase R. Lee's book on modern reloading, I also suggested after purchase, read the book. I am forced to believe no one reads the book or they do not own one because the line you refer to is on every case drawing in Lee's book.

There is no + or -, there is no SAMMI said, or says. and we do not want to forget about one of out members that called SAAMI, again; I am sure they were impressed with him but there answer was 'no, the case does not have head space.

Many years ago I ran out of answers so I asked if the person asking the question would be happy with the answer 'yes'; he responded with 'Yes' I would. It was about that time I said if you would be happy with the answer yes you should be equally happy with the answer no. And then years passed, my son called to inform me he had run out of answers. He said because 'no' no longer works, ask your mother had the same results. He added a few more responses and then I suggest he asks his children if they would be happy with the answer 'yes'. It was about that time he told me that made no sense then and it makes no sense now. I explained to my son he should be trying to get them to think, getting them to think buys time.

F. Guffey

[Yosemite Steve]

I think I undetstand. Datum measurements i.e. to case head and bolt face give the reloader a definative number so that there is no guess work in matching a cartridge to a chamber. For me, without datum measurment, there was some guess and check in finding a good size adjustment to get the cartridge to fit the chamber closely. If I took a datum measurement for the chamber I could then measure my case and get it's datum. I still have to guess and check though to get the case just right. Is there such a thing as a reloading die that you can set to a certain headspace?

Maybe Guffey could explain exactly what he uses them for and how so that his rants about them might enlighten the rest of us poor deprived souls.

[F. Guffey]

I've read through this thread a couple time's and hove absolutely no idea what this is about or how it would effect handloading.

It is about the infatuation most reloaders have with 'head space'. If I chamber a round and find the bolt will not close I want to know by 'how much', I do not need a head space gage, I do not find it necessary to measure the length of the case from the shoulder to the case head. All that is required is 'shop skill' and gages.

I measure the length of the chamber from the datum/shoulder to the bolt face three different ways without a head space gage in thousandths. And I will add; not one of the three methods and or techniques involves a head space gage and shims. I no longer offer to modify head space gages (go-gages) to go -to-infinity gages. It has nothing to do with those that understand, it has to do with those that would forget where the ideal came and the bad behavior displayed on the Internet.

F. Guffey

[hounddawg]

You discovered?' I have suggested reloader purchase R. Lee's book on modern reloading, I also suggested after purchase, read the book. I am forced to believe no one reads the book or they do not own one because the line you refer to is on every case drawing in Lee's book.

I am sure that somewhere out there is load data for .500 Nitro's and .225 Jaws Micro Magnums but that knowledge also falls into the realm of why should I care. I have absolutely zero need to know about belted magnums or .500 Nitros or .225 Micro Magnums. If I ever should somehow come to own one of those calibers and want to load for it then I will research each and every aspect of it. Until then it is simply nice trivia to strut the epeeny but has zero impact upon the reloading tasks I perform at this time

[Yosemite Steve]

I measure the length of the chamber from the datum/shoulder to the bolt face three different ways without a head space gauge in thousandths.

In the voice of Milton Waddams.

[mehavey]

Good Grief....

[Dufus]

The only cartridge that I am not concerned with head space is the 700 Nitro Express. All others fall in line for the measuring stick.

[StripesDude]

Sorry guys, I would have jumped into this discussion and added my 2 cents on datums (or whatever it is certain folks are pontificating on today) but I like spending time loading rounds and shooting them, not diving into semantic ridden discussions with condescending Internet forum members.

All that matters is how well you shoot. I’d like to see some folks post their targets...the folks who can measure better than the rest of us, and who have a PhD in reloading vocabulary. Are you putting up good groups? If not, none of this matters.

I always like reading this forum, but the condescension needs some moderation.

[F. Guffey]

There was a socially dysfunctional forum that was shut down, picking them out is not difficult. It was as bad as the old Culver Shooting page. And now CSP is on their 3rd go-around. After the first go around they dumped the archives. I do believe if someone is going to be involved they should display their pride, with no self respect or pride they cheat other members.

F. Guffey

[Yosemite Steve]

What StripesDude said. Self respect and pride are fine if you keep it to yourself. What you do says who you are, not what you say unless it's annoying.

[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.