I remember us having a discussion about this cartridge's capability 2-3 months back. I'm delighted it's come up again. Let me say first off that the barrel on your dad's rig is probably one of the fastest in the world right now! Mean initial velocities from a 27.5" barrel chambered for this cartridge of 3275 fps with 180-grain projectiles is truly extraordinary! This is as fast as any .300 WM will drive them using a barrel of the same length! As for Boyd's load, a 210- grain projectile at 2900 fps is also really moving out for a cartridge of this case capacity. I'm impressed!
The rifle I own in .300 WSM really seems to fancy Lapua's 167-grain Scenar, which it will drive to mean initial velocities of 3325 fps in typical, Houston-area, summer day conditions. Strangely, however, its efficiency is not proportional with heavier bullets. Velocity drops off decidedly in bullets weighing > 175 grains. For example, it cannot drive the Lapua 185-grain Scenar under the same forementioned conditions any faster than a mean MV of 3050 fps - a drop of almost 300 fps from a bullet by the exact same manufacturer and of the exact same design, weighing only 18 grains more.