I received a chamber cast today, from a Remington 798 375 H&H. BTW, I enjoy looking at chamber casts or chamber impressions, it is educational.
Here are the raw measurements. Contrary to what some sources claim, cerrosafe expansion is not 100% predictable. In my experience an old casting is probably 0.0015" - 0.0020" larger than the true diameter, so let's subtract 0.0017" from the cerrosafe diameters.
groove diameter, 1/2" beyond chamber neck: 0.3789", 0.3790", 0.3791" cerrosafe, 0.3773" true diameter.
bore diameter: 0.3690" cerrosafe every time, 0.3673" true diameter.
length between chamber neck-to-rifling: 0.220"
throat diameter: starts out 0.389" and tapers to groove diameter, 0.3873" true diameter
chamber neck diameter: 0.4110" cerrosafe, 0.4093" true diameter.
The customer proposes a 0.378" diameter bullet. I neither agree nor disagree, since only trial and error can determine the most accurate cast bullet diameter.
It is typical for the 375 H&H to have a very sloppy throat. Since It is not practical to size the bullet to fit the 0.387" throat, the bullet must take an unsupported flying leap. Not the end of the world, but don't expect to set a new benchrest record, either.