Roy Weatherby introduced this extremely powerful cartridge in 1953, and at that time paired it up with the 1917 Enfield and later the Brevex Magnum Mauser action. By 1958, he had his own Mark V Action that easily held the cartridge and two below it in the straight-feed magazine.
The 378 Weatherby Magnum was intended to replace the 375 Weatherby Magnum, which did not yield enough additional power over the 375 H&H. The 375 Wby Mag is simply the 375 H&H fire-formed to greater capacity by straightening out the cartridge walls. The 378, on the other hand, is a much fatter and somewhat longer case; inspired in size by the 416 Rigby.
The 378 Wby has a reputation for having horrendous recoil. I does have plenty, but not as much as some of the exaggerators claim. It's as if one after the other tried to top the previous guy in his use of adjectives. However, make no mistake, it is much more pleasant to shoot with a muzzle brake in place. Those who whine about muzzle brakes should be advised that hearing protection should be used when shooting powerful rifles (or when observing) whether the rifle has a brake or not.
A few years ago, an extensive test was conducted using three dBA meters placed at different distances and angles from the muzzle of a 300 Winchester Magnum. The brake was removed and installed while the rifle was repeatedly fired and the meters moved to new locations. The findings were that when the meters were set back about 40 yards, the rifle produced the same level of report with or without the brake. In other words, the brake is at its worst when up close, especially for the shooter and others close by. Put differently, almost anyone wanting to use a 378 Weatherby effectively will need to use the now-standard Accubrake, and they will need effective hearing protection.
Imagine shooting 270-grain bullets at 3100 fps, and 300s at 2900 fps. This is huge power. Today, some well-constructed, premium-grade bullets will stand up to the tremendous impact. In times past there were many bullets that wouldn't. Even Woodleigh has come to the party with their 350-grain round soft nose in a heavy-duty version. They can be driven in the mid- 2600 fps range, and are as deadly as Kryptonite. When it comes to deep penetration in the toughest game with an expanding bullet, I doubt that any other 37-caliber bullet is as effective.
Not only is the 378 Weatherby Magnum capable of taking a T-Rex, even from a safe distance, in the Mark V Deluxe it is a very pretty rifle. Even the Weatherby bashers, who hate them because they can't afford the one they want, secretly lust for a 378. I have admittedly shown off with mine a bit, by punching holes through a 30-inch standing maple tree, etc. I have never used one on a hunt, but have relied on the reports of other honest men to realize what a beast it is. Because of its great power and flat trajectory, suitable for hunting just about any big game anywhere, it has been referred to as "The Ultimate Rifle." In a side-by-side comparison, the 338 Lapua is a limp-wristed wimp.
Unfortunately, 40-caliber rifles and larger are required for hunting some species of dangerous game in various countries in Africa. At 200 yards, the 378 Weatherby Magnum has as much energy as some of the "legal" rifles at the muzzle.