You'll have to back that statement up with actual quotations, please. i.e. on the .318 WR.

The military .303 fmj ammo was hopless.

The .275 Rigby FMJ penetrated well.

Later his preferred rifle was the .318 Westley Richards and the 250 gr FMJ ammo.

The .275 was chosen for light recoil when shooting from a tripod in high grass on undisturbed here's in the Karamoja. Later he preferred the .318, different conditions.

My recollection it was not "ammo failures" but penetration issues with the pointed .303 fmj ammunition. I don't remember any mention of .318 ammo failures.