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Old 11-10-2019, 11:30 AM
Unclenick Unclenick is offline
Join Date: Nov 2010
Posts: 1,110


The idea behind it being a "headspace" gauge is that by measuring a fireformed case you are making what is called a transfer measurement of the chamber. That is, you transfer the size of your chamber to the measuring tool using the fired case as the transfer gauge.

In practice that doesn't work perfectly because unless you have an over-pressure load that stretches the chamber, the case is going to spring back a thousandth or two and your reading will be short of the actual headspace dimension. To get an accurate indirect measurement of headspace, you have to fire the case, make a reading, neck size it only so the sides of the case don't make any die contact (which changes its length) and fire again and measure again. You keep doing this until the case is just getting snug chambering. At that point, you can figure your last measurement before that was pretty darn close to actual headspace size.

It's a lot of work, and just removing the ejector and extractor from the bolt and shimming your GO gauge until you just barely feel it touch the shoulder and subtracting a quarter to half a thousandth is at least as accurate and doesn't require all the shooting and measuring. So I don't think the RCBS tool is really a very good system for measuring headspace. What you can do with it, though, is figure that a single fireformed reading (no neck sizing-only shots afterward) is at least a thousandth short due to spring-back and that if you resize that case until its chamber-filling dimension (bottom of head to the shoulder at its datum diameter) is -0.001" shorter than you read on the gauge, it will be at least -0.002" shorter than the chamber actually is, and that is enough for reliable feeding, as demonstrated by the fact a lot of commercial ammo is about -0.002" shorter than a minimum chamber and still feeds into a minimum chamber reliably.

The reason the gauge zero is on the minimum chamber number has to do with how SAAMI uses dimensions and tolerances. SAAMI uses an ANSI Standard engineering practice for critical dimensions. "Critical", in this context means a dimension beyond which the device cannot be assembled to function. We are accustomed to seeing non-critical tolerances of + or - some average number. These are used when it makes no difference whether an error is over or under. But with critical values, one end of the tolerance range is an absolute limit while the other end is more flexible. In these instances, the dimension given is the critical limiting value, followed by a unilateral tolerance (minus only or plus only) in the non-critical direction. For SAAMI, a cartridge's critical dimensions are maximums beyond which you cannot close the gun on them, where undersize amounts are less important to make exact. So, for example, the 30-06 head diameter is 0.473 - 0.010". That means 0.473" is the critical maximum size that is still guaranteed to fit the tightest SAAMI-dimension chamber. It is the critical dimension. But you can make it smaller down to 0.463" and even if you went over and made it 0.460" it would still fit into the chamber, even if it might not fire or extract reliably on some guns. That is why the minimum size is not critical. For lengths expressed without a tolerance, the drawing tells you to assume -0.008" cartridge tolerance, but all those dimensions given without a tolerance are still critical maximums.

The chamber is just the other way around. For the chamber, the critical dimensions are minimum dimensions, below which you might not be able to chamber a round. So the chamber dimensions given are critical minimum values with a plus (+) tolerance. Even though the drawing explicitly shows the minimum and maximum headspace dimensions, in particular, that is just to prevent a gunsmith making an arithmetic error. The operating principle is still that the chamber's minimum dimension is the critical one to the gun being usable, while the tolerance is +0.010"*.

The bottom line here is that the RCBS gauge is zeroed on the chamber's critical headspace dimension, which is its minimum. That is the reason for choosing that end of the range for the zero point on the scale.

By the way, if you chamber your rifle with a pull-through headspacing reamer, you can be pretty sure of hitting minimum headspace closely. Your first fireformed cases fired in that new chamber will give you the amount of spring-back your loads have below chamber minimum and you can record that information with your rifle file to monitor changes in its headspace in the future.

* Note that the industry standard NO-GO gauge is usually just +0.004" or +0.005" for new chambers because they figure the size will settle in, while the industry FIELD REJECT gauges are often +0.008". I don't know why that deviates from the SAAMI standard other than the gauge makers want some wiggle room for their gauge tolerances. The military will accept up to +0.015" in the field before it rejects a .30-06 chamber.

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