The .375 Ruger has a .532" base diameter, which measurement is shared with the .30, 7mm and 6.5mm PRC ctgs., thus, they are based on the pre-existing .375 Ruger case.

The belt diameter of the standard magnum cases, is .532". The head diameter is a mere .513" give or take a thousandth of an inch.

PRC and .375 as well as .416 Ruger brass can be made from magnum brass, by merely trimming the case length appropriately, then necking
to the correct bullet diameter, making the shoulder in the appropriate location for headspacing and fireforming the case. The case body ahead
of the belt will blow out to .532", conforming to the PRC or Ruger chamber.
Note, inside neck reaming might be necessary, especially for the 6.5, 7mm and .30 cal. chambers.
This same method was used by me to form 6.5 & 9.5x68 cases. I turned the belts to .522", trimmed and necked the cases and fireformed.