Karl,
I have had plenty of 378s. They are 3150 f/s with 270 grains and a bit over 3000 f/s with 300 grains. That with Re22, 2213 and the old original 4831.
For Americans. 2213 is the earlier version of what Hodgdon sell as H4831 SC.
The 378 in 26" barrel is virtually identical in velocity to the 300 Winchester in 26" barrel with bullets of the same sectional density, a 270 grain is same as 182 grain 30 and the 300 grain is same as 202 grain 30 so the 378 is just a tad faster than the 300 Winchester.
I have chronographed over a large spread of years the 270 grain factory load which has always looked like the 270 grain Hornady and velocities have ranged from a low of 3080-3100 to a high of 3150-3180. I have never chronographed or even shot the 300 grain factory load.
Here is data from AR's reloading pages and it's Mitch Carter's loads. Mitch was the king of shooting the 577 T Rex.
http://www.accuratereloading.com/378wby.html
