I had a .300 mag I chambered up on a Model1917 Mauser. As originally a Model 30 Remington, and a slow speed one at that, 200fps lower than Speer book specs, it turned into a fine performer after the rechamber.

I found it absolutely loved 165gr. Nosler Partitions and 165gr. Winchester Fail Safes, delivering 3,260 with the Partitions and 3,160fps with the same load and the Fail Safes.
Max Speer load with IMR7828 - about 82.0gr., IIRC.
Both were 1/2" to 3/4" groupers.