Made the decision, took the plunge, and ordered a new reamer from Pacific Tool & Gauge.
Back in early 2003 I imported a ZKK-602 in .375 H&H from Kamloops B.C. I then had a fellow in Prescott AZ rechamber to .375 Weatherby about three months before Weatherby re-introduced the cartridge. I could never get the rifle to hit the target.
Last year I sent it to Jim Brockman to see what he could to with it. He found the receiver had been twisted 0.060" out of alignment. He milled Talley rings to take this into account, and did some other work, the end result is now I can make 3" groups with my best loads. No joy on that.
I have another 602 in .375 that I was going to build up from scratch, but decided to see what Jim could do for me with a good reamer. I think the problems with the other rifle are due to the enormous chamber, and the original Weatherby free-bore which is 0.750" long! Combine that to the very easy entry to the rifling of 0.261" and a bullet isn't fully engaging the rifling until 1.0105" of virtual free-flight down the bore.
This reamer will be a "match grade" reamer for a very tight chamber.
The following changes:
Neck diameter: 0.405" to 0.401", which is 0.001" over the neck diameter of two full boxes of original 1958 .375 Weatherby cartridges I have.
Cartridge case mouth to free-bore: 0.0145" to 0.010".
Free-bore diameter: 0.376" to 0.3754".
Free-bore length: 0.735" to 0.125".
Throat (length of entry into the rifling): 0.261" to 0.100"
Angle of rifling entry: 1*-19-0 to 1*-30.
While this isn't a target chamber by any stretch, I believe it will be a significant improvement over the sloppy factory chamber, while still having more than enough room to seat-out long mono-metal projectiles.
Once the reamer is here I am going to send the rifle and reamer to Jim in Idaho and have him work his magic. Besides the rechamber he'll float the barrel, pillar bed the action, do some action work and re-cut the barrel crown. It'll look like a plain-jane 602, but I think she'll be able to place the shots where I want them.
Last thing I'll do is have a Wilson-type micrometer bullet seater made so I can get the full benefits of the better chamber. We'll see.