Datums for Dummies

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You know what? I've hand loaded for 50 years w/o much need to worry about datum this or datum that. I've used several methods of finding and/or adjusting the cartridge to chamber fit w/o need for hundreds of words beyond what I found in an old Lyman 1970(ish) reloading manual.
 
Jim, I'm sure you know this, but that's actually head clearance you are referring to (also in the SAAMI glossary). Headspace is the total space for a cartridge to fit into, so it includes the space occupied by the cartridge from breech end to the surface that stops on the chamber headspace determinate. Head clearance is what is left over after you subtract the space occupied by the cartridge from the headspace in that area.
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Well, I'm going to step up to the plate and admit that I too confused headspace with head clearance!! :eek::D
 
Quite frankly I am about datumed out myself. So far I have managed to pull and replace barrels and bolt faces several times on my Savages without destroying myself or a gun. I know how to set a shoulder back and I am pleased with my loads performance so I think I have a good enough understanding of head space here for my needs at least.

so with this I am out of comments for the thread so have fun discussing how many datums can break dance on the head of a theoretical needle
 
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.
 
Everybody who's anybody knows that ammo doesn't have headspace. Hrrrumph.
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Hrrrumph Indeed...

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

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