I know this is an old post, but being new to this board I'm diggin in the archives.

It is possible the chamber was headspaced correctly but the neck length was too short. This could be for a number of reasons. Some Smiths save money buying a go and nogo gage for a 30/06 and a complete reamer for a 25/06 Along with this they buy a neck and throat reamer for 6.5mm, 270, 7mm, 308, 8mm etc.. With the 25/06 reamer you could ream and headspace a 270 winchester chamber and go in afterwards with the 270 neck and throat. The smith could have made his own ammo using 30/06 brass and never had a problem with excessive pressures. Factory 270 winchester is based on the US1903 30 cal brass and is longer than the other cartridges that use the same headspade gage. A complete 270 reamer could have been ground with the neck too short. Both these situations could have caused the neck of the brass to crimp the bullet enough while chambering to cause excessive pressures enough to cause the problems you had. You're lucky it was a Mauser 98. If it was with only one brand of ammo I would have thought ammo. Do you have any of the fired brass from that chamber left?

gunmaker
http://users.elknet.net/chico