Gunplumber,
Yes. The Internet age has changed it all. Lot's more available.
As you can imagine, since some cartridges are narrower than 3/8", you can't find the shoulder with that number on them. Similarly, some cartridges have bullets wider than 3/8", so you couldn't find a shoulder number at that diameter on them, either. Here are some drawings from the SAAMI site. The upper is the cartridge, while the lower is the chamber. Note the line through the shoulders of both have the same diameter number, with the letter "B" following. That B is for Basic, meaning it's a defined dimension, so it has no tolerance, and is a number other measurements are referenced to. In other words, a datum.
460 Weatherby Magnum, 0.5300" datum diameter.
375 Remington Ultra Magnum, 0.465" datum diameter.
308 Winchester, 0.400" datum diameter.
30-06 (and all other 30–03-based cartridges, IIRC, including 280-AI and others, 0.375" datum diameter.
222 and
223 Remington, 0.330" datum diameter.
218 Bee, 0.2875" datum diameter.
If you go to the SAAMI site to look up individual cartridges,
here, note that the site has a technical issue (which they mention) in that this page's links to drawings only work in Internet Explorer and no other browser.
Note that the tolerances in SAAMI drawings are unilateral. This is a common engineering practice where getting tolerances too large in one direction is critical to operation and the other direction is less so. Most of us are accustomed to the familiar + and – tolerance, where the value given is in the middle of the range, but that's a situation where being bigger or smaller impose equal problems. For unilateral dimensions, you give the critical extreme value and then give the tolerance only in the other direction. For example, a shaft that's too small for a bearing will vibrate and wear itself or the bearing fast, but a shaft that's too large cannot even be assembled into the bearing. Thus, too large is the critical dimension, and so shafts are specified as a maximum diameter with only a minus tolerance for proper operation.
In the case of the SAAMI drawings, lengths and diameters and inside corner radii of cartridges cannot be allowed to get too large or they won't fit in a chamber. So all those dimensions are given as maximums and the tolerance is minus only. An outside corner radius is a minimum with a + only tolerance, as shrinking an outside corner, like the shoulder and body intersect corner, can jam in a chamber. For the chamber it is the other way around, with the minimum space being critical to being able to chamber a round. So chamber linear dimensions and outside corners, like the neck and shoulder intersect, are all minimum dimensions with plus tolerances only, while inside corners, like the neck and shoulder junction for a chamber, have radii that are maximum dimensions with minus tolerances.
One funny result of this is most SAAMI cartridge maximum dimensions are bigger than minimum chambers because it is still possible, allowing for the case to expand to the width of a minimum chamber, to force a fit. However, nobody intentionally manufactures cartridge cases over minimum chamber size on purpose. That happens when they expand into a headspace that is larger than minimum, but don't come that way from the factory (remember that factories want their ammo to fit all guns). The military, though, doesn't play that game. They want to be sure there is no "squeeze" that might slow full-auto fire, so NATO chamber minimums equal cartridge maximums.
Measuring from the breech to the body/shoulder intersect is the old way used by Hatcher and, apparently the CIP, since they don't provide a datum. It works, but has two limitations. First, the body/shoulder corner in a real chamber has at least a microscopic radius, so the actual intersect is buried in the metal. Additionally, with a shallow shoulder angle in particular, there is no clear ledge for your measuring tool to meet, so it is difficult to get an exact measurement if you want one for a minimum chamber. That is why we use headspace gauges.
The second drawback is that because the cartridges are narrower than the chamber, when a cartridge is seated against the shoulder of a chamber, its own body/shoulder intersect is slightly forward of the chamber body/shoulder intersect, so it's a different dimension. The datum diameter is identical for both cartridge and chamber, providing a common reference point.
Machineguntony said:
What are the consequences if the headspace is wrong or off?
In machine guns, it is as described already.
In a bolt rifle with a closed bolt, the consequences are only to brass life. First, if you closed the bolt on the cartridge, you already know the headspace isn't too short to allow chambering. That's number one. It can, however, be somewhat short and the cartridge squeezed in, which raises pressure a bit, but not beyond SAAMI tolerance for individual cartridges in a test set of ten fired to determine Maximum Average Pressure (MAP)¹.
Number two is too long a headspace. Hatcher''s Notebook details an experiment on 30-06 with a special reamer that let them enlarge the chambers by 0.050" (IIRC) and they kept firing and the case shoulders no doubt blew out too long to resize easily, but beyond a point the extractor claw won't let the cartridge go any further forward, and fi your firing pin reaches that distance, it still fires fine. Oddly, he noted an actual increase in velocity as the cut the chamber incrementally deeper, up to a point. I've never see that explained satisfactorily. You would expect the opposite to happen.
I was at the range one day when a fellow walked up to me holding a case with no neck and asked me if it was normal. A friend had lent him his "308 rifle" to hunt with. He bought 308 Win ammo for it, fired one round, and extracted a case with the neck blown out all the way to the mouth. Quick inspection of the barrel stamping showed that it said "30-06". A 308 Win shoulder exceeds the 30-06 chamber minimum diameter by a little bit, but when you combine the plus 0.002" chamber diameter tolerance with the minus 0.008" cartridge diameter tolerance, you get parts that can just squeeze together. The bullet is the right diameter, so it is expelled, but velocity would be low and I would expect a lot from accuracy. But no damage was done by all that excess headspace. It just ruins the case.
Slightly excess headspace lets cases stretch and thin the pressure ring grows thinner faster. That reduces how many reloads you get before the case heads start to separate.
¹ We are used to looking at that number as a pressure limit for individual cartridges, "the SAAMI maximum", but it is actually for an average peak pressure of ten rounds with a standard deviation of 4% of value, and individual rounds within that average are allowed to vary by another number called the Maximum Extreme Variation (MEV), which is a pressure range. In the worst case (virtually never happens), nine of the ten have pressure peaks a tenth of the MEV below Map, and the tenth cartridge has a MAP nine tenths of the MEV above MAP. That works out, the way the pressure statistics are done, to about 18.3% over MAP (varies slightly with individual rounds due to rounding error) for that one cartridge. It is still below minimum proof load levels. CIP allows a hard limit of +15% for individual rounds in an average to go above their MAP.
