I would suspect that the previous owner may have a quantity of fired brass which can simply be neck sized and reloaded. No problem.
IMO, if the neck dimensions, overall cartridge length and powder charge are the same for both 318WR variants, there should be no issue with firing the standard 318WR cases in this chamber. And this is the easiest way to get well formed brass for the handloading process which would eventually be the remedy to your situation.