The problem with the factory Weatherby in the 30/378 is three fold.
#1 There is too much free bore for one thing.
By the time you seat the bullet out to TRY to touch the lands, you will never be able to use the magazine because of the extended length of the case. You may never be able to touch the lands anyway.
#2 The barrels need to be longer to shoot the heavy bullets with a high BC.
They advertise a 28" barrel but they actually have a 26" barrel with a muzzle brake
. As we all know, the muzzle brake does not add barrel length. As a matter of fact the bullet does not (or better not) touch any part of the brake. There should be .016" free space completly around that bullet when it passes through the brake.
Therefore, it is not part of the overall length of useable barrel.
#3. Check out the bolt locking lugs on a factory Weatherby action sometime. See if they are all locking evenly. I'll bet they are not. A gunsmith can correct most of these problems to make the gun shoot.
There are ways to accurize the Weatherby rifles chambered in the 30/378 to make it shoot but, right from the factory they are known to be VERY inaccurate.
Cut the barrel back a few threads rechamber and get rid of the excessive free bore is a start. Put a new custom LONG barrel on it is another way.
All my custom 30/378s had at least 36" barrels on them.
Howard Wolfe, the noted gunsmith and his whole shooting family at Williamsport shoots nothing but the 30/378 caliber Heavy guns with long barrels and 220 and 240 gr bullets.
He has shot this caliber in the 1000 yard maches since he designed it many years ago.
Why have a longrange rifle with a chambering of a big overbore case and shoot only light bullets in it? Longrange shooting or I should say Longrange hunting should be done with the most accurate and highest BC bullet available in the caliber you are shooting to retain the velocity and energy at extended range.