I don't know how long of a barrel you'll put on this rig you're planning, or whether or not you're a maniac for velocity, but I've got a really nice, super accurate, mega- custom affair chambered for the .300 WSM and I'm tellin' ya, it won't shoot anything heavier than 190-grains faster than 3000 fps with its 28" barrel!
My dad has one he built using a 700 and a Pacnor 27.5" bbl. His fastest load with a 180 is 3275 fps, and seems like it may be slightly faster than the others mentioned, but it's only a 180, not a 220gr. He hasn't shot anything heavier from it so I'd just be guessing. The load that was at 3275 was at 67,000 psi on the Oehler 43 and my 300 Ultra was at 62,000 psi with the same MV. He did it with 20gr less powder tho...
Dave, thanks for the heads up, Boyd's seems a tad slower than I'd have thought too. Accuracy doesn't seem to be a problem with any of them tho. I figured with the same pressure, the Ultra was about 100 fps faster, might be that Dad has a fast bbl and the difference might average 200 fps or more considering your guys rifles.
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.