I have experienced the same thing in a .338-378 with a large powder load. It also gets worse with colder temps. Mine would "hang fire" by maybe 1/4 second at 60 degrees, but would not fire at 10 degrees.
Went up to 215M's and haven't had any more problems.
This is a .338-378 that I built. MK5, Douglas barrel, Timney trigger and a Leupold .65-20. Group is .224" Probably my best ever. I have taken one bull at 702 yards with it and a friend took a moose at 350 yards.
I have a Mark Alaskan that started as a .340 Wby and had rebarreled and the bolt face opened. Now a .338-378 with 26" plus Vias brake. Shoots Berger 300's at .5"
Weatherby's have a very long throat. Bergers will work. I shoot 300 grainers out of a .338 -378 very accuratly. They will hold .5" but Ihave to run them out to 4.05" COAL vs. 3.76" standard. Single shot only but they will shoot
Do you still have this scope? I cant get back to my account so I made a new one. I am in Fairbanks and am very interested minus the rings.
If you still have it give me a call 907 460 9986
Clint