Fat chamber with excessive headspace is my guess.......find a gunsmith with a go/no gauge.
The issue with the conventional Belted Magnum Headspace Gauges is that they headspace off the belt. That measurement is probably correct. The distance to the shoulder tapper datum in the chamber wouldn't show with conventional belted magnum headspace gauges. According to his once fired brass, that dimension is correct by SAAMI specifications anyway. His chamber diameter is somewhat excessive just in front of the case belt but again, a headspace gauge won't really identify that either. His factory brass seems to be significantly under the proper length (-0.018") to the shoulder/datum, and his chamber is a bit too fat (too large in diameter just in front or the belt).
His once fired brass should reflect all the information he needs from his chamber, to compare with the actual SAAMI drawings/specifications for a proper 300 Winchester Magnum cartridge. A chamber casting would be of little to no benefit after having once fired brass from his chamber, to measure.