That is easy... his rifle(s) were action and full length bedded in fiberglass - essentually a composite stock on the inside - so that barrel heat or ambient temperature heat or cold, water or snow did not move move the barreled action changing POI. And as everybody else has indicated the outside was sealed tight with a finish.

I have an old PO Ackley 30-06 Improved that was done the same way by Ackley himself. If I had a wood stocked hunting stock - it would and did get the same treatment - with a composit not necessary if the barreled action is bedded correctly. It would never have a floating barrel as guns do today.

Remember the pre-64 model 70 the O'Connor used even had a screw way up in the forearm that tied the barrel to the forearm - no movement at all