what muzzle speed may I expect shooting 180gr factory ammo through the average out-of-the-box 26" barrelled rifle in:
300 winchester mag
300 weatherby mag
300 rem ultra mag
30-378 weatherby mag
Thanks in advance.
Montero

Without chronographing each individual load (as well as each individual lot within a brand and load), it's really tough to say.

Several years ago, standard American ballistics table velocities were revised downward. It was said to reflect the "real world" use of 22 to 24 inch barrels rather than the standard 26" test barrel. I think that it is true that you will find the velocity of factory rounds closer to the published figures that they were 20 years ago. Ammunition manufacturers generally load their standard ammunition to lower pressures and velocities, in part to have an excuse to market their more expensive "express" or "high performance" or whatever they call their extra velocity ammo.

With a 26 inch barrel in the calibers you mention, I would expect close-to or slightly lower velocity than listed with the .300 Winchester (assuming standard Winchester, Remington, or Federal ammunition), while Weatherby factory ammunition has nearly always given up-to-speed velocities. I haven't seen any figures for the .300 Ultramag factory ammo, so can't venture a guess.

in my 700 BDLSS with 26 inch bbl. the ultra goes a little over 3250 with 180 nosler factory load. 3350 with 165 sierra 91 gr, rl 22, and 3590 with 150 sierra 96 gr. rl 22.