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Headspace is headspace. Nothing to do with freebore dimension to suit a given projectile or neck diameter for
Particular brass type....but these should comply with SAAMI nonetheless.

Jaco, do you have the means to take a chamber casting at all? Either CerroSafe or Sulpher / Graphite? And then measure the casting to confirm chamber dimensions?

Very unlucky to have both a tight neck and short throat in a ‘SAAMI’ chamber.

Consider running a neck reamer up it to get the neck clearance you need for the brass you intend to use. Seat a...