The Spanish Proof House prints a leaflet accompanying Spanish guns and it contains a diagram and all the min-max internal dimensions, including chamber length, diameters for rim, chamber, cone, bore for all gauges down to 9mm shotgun.
For thickness the steel quality comes into play, the stronger the steel the less of it required. The CIP published a chart showing minimum thickness for three ISO specified steels. This chart was reproduced in various books on proof, one of which was sold by the Blue Guide.
