I really don't know what AGI is up to with the tape, either. I've never heard of doing that. My first speculation, mentioned earlier, is that they are aiming for the middle of the headspace range for feed reliability. The only other thought I had was regarding the caution Hatcher had about closing bolts on headspace gauges. He points out that many bolts are perfectly capable of camming hard enough to stretch the chamber several thousandths with a headspace gauge inside, rather than refusing to close. So perhaps AGI is also trying to make an allowance for somewhat ham fisted new practitioners. A little practice measuring with a micrometer gives you appreciation of the need for feather light contact in accurate measuring. You simply have to develop a feel for it. But I like my first speculation on AGI's purposes better.
BTW, the light touch is why one is normally advised to strip a bolt before trying to use a headspace gauge. Even if you have an ejector notch in the gauge, spring pressure from the cocking mechanism can interfere with how sensitively you can detect first contact between the gauge and chamber.
Mr. Guffey is correct that you can either get other chamber dimensions to deduce effective headspace from or you can ignore exact headspace if you are going to load only cases fireformed to your chamber and set back only as needed to function reliably. Just don't fire commercial or military loads in an undersized chamber, as that increases pressure. If in doubt, put a squib load together and fire it to see how the case length comes out.
Hatcher reported on some experiments where .30-06 headspace was intentionally extended 0.050" long with a special reamer, and it didn't damage the brass or fail to shoot well enough for military purposes. The standard new rounds going into the long chamber just headspaced on the extractor hook and blew their shoulders forward when they fired. Lots of wildcats have been made that way. The Ackley Improved approach is probably best known among those that will allow you to shoot the parent cartridges directly without first knocking the charge down or using some kind of blank load to form the cases. The only thing is, you then want to resize for your cases for that big chamber or the brass will thin out at the pressure ring and start giving you head separations in relatively few reloads. Also, accuracy won't be best if the case rattles too loosely in the chamber.
There are lots of ways to work out there. Ultimately, the main thing is to keep in mind what your objectives are and to work out how to get yourself there and prove to yourself that you've gotten there. Headspace gauges are a conventional approach and are also used in manufacturing to meet SAAMI specs, but it's worth noting that SAAMI is oriented toward manufacturing and that an individual with time and different tools and a bit of ingenuity can find lots of workarounds for conventional methods.
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Last edited by Unclenick; March 12, 2012 at 10:11 AM.