Jimro, I knew he was referring to the converted .303's, but 'twas probably good for others reading this threa that you mentioned that.
None of the M1 or M14 rifles had relief ports when I was shooting such heavy loads. I don't think that such modied gas system were even thought of back in the '60's and '70's. The extra loud ring of the metal parts being smacked extra hard by moving parts told the shooter a heavy load was just fired.
US Navy Distinguished Marksman Badge 153
Former US Navy & Palma Rifle Team Member
NRA High Power Master & Long Range High Master
NRA Smallbore Prone Master