“Only the Lee Modern Reloading, both editions, shows the datum dimension.Sierra, Lyman, Hornady, nothing. If you have a small metal lathe, you can make these gauges. That's what I do. I know the "experts" will go berserk, but I measure the datum line on my ammunition and compare it to the spec. If it's within a .001" or so, that becomes my headspace gauge”
I never know who I am allowed to disagree with:
I collect datums, I purchase datums, I make datums, I purchased two boxes of what appeared to be junk and miscellaneous parts, old parts at the Big Town gun show in Mesquite, TX, 40 lbs. $20.00, every old part in the boxes had a hole in it, attached to the holes were dies, case holders for case trimmers, etc.. to make the deal worth while there was a RCBS collet bullet with a 30 cal. collet ‘stuck’ in it. AND! There were Lee dies, to save weight I started sorting/separating the Lee dies in an attempt give them back (for free) The dealer would not allow returns??? He claimed he threw the Lee dies in with the deal?
Point? A lathe is not necessary, R. Lee’s book on modern reloading is a source of information for the diameter of the hole to be drilled in a flat surface, SAMMI is the original source. The datum/hole for the 308 W is .400 in diameter, the datum/hole for the 30/06 is .375 (3/8”). Again, I am not politically correct, I can use the .400 datum/hole for both the 308 W and 30/06, it is much easier to find the .3/8”/.375 hole than the .400” hole, I do not insist the diameter of the hole match Lee and SAMMI references. I make DATUMS, my chambers, my cases, All I have to keep up with is the index .000’.
A lathe is not necessary, nice but not necessary, the diameter of the hole? Not necessary to match LEE/SAMMI specifications unless the owner of the hole wants to compare the length of the case/chamber with LEE/SAMMI specifications.
Short cut: Fire formers chamber a case of unknown length and then pull the trigger, the case when ejected becomes a transfer as in transferring the chamber dimensions/length to the datum based gage. Had the fire former measured the length of the case before firing from the head of the case to the datum? then compared the length of the fired case with the minimum length/full length sized case they would know the effect the chamber had on the case when fired, This works for all but those that have cases that jump back, snap back or spring back, hammering my cases causes them to loose their memory, my cases have little to no recall as to what they were before being hammered from the inside out.
F. Guffey