TX1200 match barrel if a PH? That's a suggestion. Mine was marked TX1200.

14" twist sounds suspiciously like it could have been meant for the 147gr. CDN or Nato round.

Have you mic'd the bore and groove? Calipers are not accurate enough - if it mic's .307" groove or .3065" it just might be for that mil-spec ammo - however 14" still sounds slow.

In 14", it should shoot up to 140gr. normally shaped bullets, but I'd try the 125gr. TNT Speers (beautiul bullet) or light Sierra match bullets.

It might even shoot a 150gr. Spitzer, but maybe not. Only one way to find out.