"I respectfully disagree. The .308 family of cases headspace is very relevant. Headspace is measured off a datum on the shoulder therefore would you use a 223 mic to measure, absolutely not."
One, you were specifically addressing the RCBS "head space" gage (it's actually a case gage; headspace is in the chamber) of the .308 family so I didn't think it necessary to warn you of the impossiblity of trying a .223 or .30-06 RCBS Case Mic to check your .260 ammo. (Seems I was wrong tho!)
Two, new ammo isn't made to be a headpace gage so you certainly won't get a 'zero' on much or any of it. New cases are always made slightly smaller than minimum cartridge size to insure it wiill chamber in anything so no matter what number you get on new stuff it will be meaningless in application.
Three, the datum line is meaningless to anyone but a chamber or die cutter.
Four, all of the .308 family use a .40" datum so it won't matter what RCBS Case Mic thimble is used for any of them. Fact is, reloaders only need a repeatable point anywhere on the case's shoulder to serve as a comparitive (not absolute) reference so he can properly adjust his FL sizer die. Thus, actual headspace, as such, is irrelivant to a competent handloader; ditto an incompletent one for that matter!