I took a 223 wssm barrel off a model 70 put a 243 wssm barrel on loaded a new brass 35 grs varget with a 100 gr bullet fired it and the gunsmith who put the barrel on measured the new brass and then the fired brass and he said 8 thousandths the fired brass looked good
SAAMI website should have specs. I believe it was .010" for 25-06 between go & no go. I had a 25-06 with .012" from the factory. I'm going to guess that there's a large percentage of factory rifles on the fat side of headspace. I have a custom with around .006" and it shoots bugholes.
Just came from the SAAMI web site (www.saami.org). Their drawing shows 1.2403 min. and 1.2503 max. for the Win. 243WSSM. That s "GO" to "FIELD". Exceeding "FIELD" invites dangers conditions. Me? If I had "GO" + .008, I'd set it back a few thou. Say, "GO" + no more than .005",, preferably GO + .002-.003".