I agree, its most likely that your cylinders are cut to min. specifications and the ammo your buying is not...
Many customs in this caliber keep the dimensions at minimum anticipating there owners to push the limits of loadings. Its a long time benchrest trick, bore them tite, and burne'm hot and make little bitty holes at long ranges... Barrel life span to them (Bench rest shooters) is irrelevant, accuracy is the only concern.
My advice, Start hand-loading!!!!
CW