think its called datum line ( memory?) usually SOMEWHERE near middle of shoulder ....object is to end up w/ abt .002 clearance between bolt face& rear of max.cartridge for tight hdspce .....on military sprgflds , they went back to armory somewhere between .006 & .010 .....however every military mauser i checked(many) was over .010, & several russian 7.62 x 54 also....soooooo go figure .....mcfarland wrote commercial mausers exported to USA were also .010, but that the germans didnt mind killing a few civilians between wars!!.........these datum lines are on cartridge book drawings & on some of my old reloading manuals ......incidentally ,u can make ur own hdspce ga. quite quickly ...turn up a piece of steel to fit rear of chamber w/ a shoulder the thickness required ( max case rim + .002 )....for rimless turn up a piece of double diameter, chamber d. at rear & DIAMETER of datum line for short section in shoulder area.make it the stated length fron datum line to rear end of max cartridge + .002 ...these are go gages .....004 more for no go ..BE careful using rimless ga. ,that u dont chamfer/ROLL the edge over as it is not hardened....have never barreled up a belted ( reckon no one thot there was anything big enuf in ky./ohio/ ind. to warrant one 40 yrs ago ) , but think rimless ones hdspc on datum line also .......if u are picky abt rimless ga., set up the sine bar on ur compound & turn up the commercial type ga. w/ full shoulder ....if really picky , do same w/ O-1 abt .010 oversize , torch harden , draw in kitchen oven @ 375 for 1 hour & toolpost grind to spec....have done it all ways & first one works just as well for me ......w/ bolt design having unsupported case heads ,if u have an incipient head fracture that lets loose , tight hdspce "machts nichts" ..better have a gas escape hole somewhere in receiver( & a pair of glasses ) i have been on the firing line when an action let loose three points away, & dont ever want to see( or hear) another .....
FWIW last summer ,put a 308 sporter barel on a mauser ..no ga. & NO TIME ..transportation to texas LEAVING ....grabbed 1/2 dozen pices of new brass ,dab of grease on cartridge end , some .002 shim stock & chambered so it would take any of the six .....it was checked in texas & was .001 over minimum hdspc...( nephew never has sent me a check for the cost of the barrel ! ) point being , u dont always have to have all the "NECESSITIES " to do a workmanlike job (& maybe ,get paid for material before delivering freebie labor)
best wishes
docn8as
PS THIS DOES NOT MEAN GET SLOPPY , but hdspce bugaboo has been badly overkilled by "sports pundits"(also damascus shot barrels ....that ought to stir up some dust!)....if EVERyTHING else is ok ,modern actions will stand a lot of hdspce abuse