158XTP,
After I dragged that 1984-model Mk V Deluxe 460 WBY from Missouri to Alaska to Florida and back to Alaska,
I traded it in on a BRNO ZKK 602 .375 H&H that had a 22" barrel.
The BRNO had a beat-up factory stock and an unfinished walnut custom stock that caught my eye.
Some old sourdough had given up on getting it finished and left it at Boondock Sporting Goods.
Even swap to the returning sucker, er, sourdough.
I returned to KY and had a TN gunsmith work on it in 1998.
I had that 22" .375 H&H BRNO rechambered to 378 WBY and had the custom walnut stock crossbolted, pillar bedded, checkered and finished,
and a Pachmayr Triple-X Magnum (1.5") was installed.
I had two boxes of the old Tiger-on-the-box factory ammo.
I fired 6 shots, 3 from each box and chronographed at 5 yards, 85 degrees F.
Factory bullet was the Hornady 300-gr FMJRN, BC = 0.275.
Velocities ranged from 2833 fps to 2886 fps.
Average = 2851 fps.
Add 17 fps to correct to MV = 2868 fps
Add 120 fps for a reasonable correction to 26" barrel length MV = 2988 fps
But the primers were cratered and the bolt was sticky, accuracy ho hum, but muzzle blast was, shall we say noticeable in the 22" barrel ?
Conclusion: Factory ammo was loaded too hot for my rifle, but might be doing the claimed 3000 fps in some rifles ?
I pulled the bullets and used the brass for handloads.
Times changed and factory ballistics backed down to closer to 2900 fps with 300-grainer.
I later settled for a CZ 550 Magnum .375 H&H re-chambered to .378 Wby, with 25" factory barrel.
Using IMR-7828, 111.0 grains, with any 300-gr bullet,
5 shots of 300-gr Swift A-Frame averaged 2878 fps, and first 3 of those went into just under 1" at 100 yards, before I started flinching.
No signs of pressure.
Add an inch of barrel and a BC correction from 5 yards to muzzle, and that load should be a bit over 2900 fps MV in a 26",
just about spot-on for 2900 fps in my 25" CZ barrel.
That is the extent of my load development for the .378 WBY.
Mitch Carter had a favorite load in a 26" .378 Weatherby.
I jotted it down when I studied 577 Tyrannosaur torque control technique with him at Long Island, NY in 2001:
300-grain Barnes X-Bullet (first generation)
Alliant RL-25: 116.0 grains
Claimed velocity: 3024 fps
Accuracy < 1" at 100 yards
