The .30-378 Weatherby Magnum is not a new cartridge, as it was developed by Roy Weatherby back in the 1950s as an experimental long-range number for our military. Since the military never “officially” adopted it, it was placed on the back burner until 1996 when Weatherby Inc. recognized a trend toward long-range, high-velocity cartridges among hunters and introduced it commercially.

As its name indicates, the .30-378 Weatherby Magnum (WM) is based on a .378 (or .460 Weatherby) case necked to .30 caliber, which is essentially the same as the famed .416 Rigby case but with the addition of a belt and the unique double radius Weatherby shoulder. Its powder capacity is huge, holding 120.2 grains of water. (The .300 Winchester Magnum, the most popular .30-caliber magnum, holds 84.5 grains of water, while the .300 WM holds 93.6 grains.) Factory loads, as offered by Weatherby, are advertised to drive a 165-grain bullet 3,500 fps, a 180-grain Barnes X-Bullet 3,450 fps or a 200-grain Nosler Partition 3,160 fps.

What is really surprising is the number of rifles being sold in this caliber. A Weatherby representative told me, “The .30-378 Weatherby Magnum is by far the best selling cartridge in our line.” Sales have even surpassed the .300 Weatherby Magnum, which has been its best selling cartridge since commercial ammunition became available in 1948. This impressive sales record requires us to take notice of this relative newcomer.

To evaluate the .30-378 WM, I obtained a Weatherby Mark V known as the Threat Response Rifle (TRR) Desert Magnum from the custom shop. Since this cartridge has a fistful of powder to burn in a comparatively small hole, Weatherby fits it with a 26-inch tube, which is equipped with a muzzle brake (Weatherby calls it an Accubrake), giving it an overall barrel length of 28 inches. This brake is easily removed and a cap is provided to cover the threads, should someone choose to use the rifle without the brake. Personally, I would like to see this rifle fitted with a 28-inch barrel without the muzzle brake, to help the cartridge achieve more of its full velocity potential. On the other hand, those who are recoil-sensitive will likely prefer it set up with the muzzle brake.

As its name indicates, the stock is finished in a desert camouflage pattern, while the stainless steel action and barrel are plated with dark titanium nitride, which is not only non-reflective but also tough as nails. The button-rifled barrel is made by Criterion (a division of Krieger Barrels), which has earned a fine reputation for accuracy. These tubes are made of 410 stainless steel for durability, an important feature when dealing with a cartridge that has the potential to erode throats of a lesser quality barrel within a few hundred rounds.

The stock is competition style: adjustable for length of pull, drop at heel as well as cheekpiece height. Most shooters will find it can be adjusted to fit them, improving comfort and possibly shooting skills (or just preventing getting thumped on nose or cheek). This synthetic stock features a solid, one-piece aluminum bedding block that extends the entire length of the action. The barrel is free floated. Out of the box, the trigger pull broke cleanly at 3 1/2 pounds.

This brings us to the heart of the rifle, the famous Weatherby Mark V action that was brought to the shooting public in 1957/58. Prior to that time, Weatherby rifles were largely built on FN Mauser 98 actions, a few on Brevex Magnum Mausers and Schultz & Larson actions. They were essentially custom rifles built one at a time. In some instances customers were allowed the option of supplying the action, and Weatherby installed a new barrel in the desired caliber and fit a high figure custom stock with near perfect inletting and hand checkering.

Examples I have owned and others I have examined were beautiful rifles. The problem was that relying on other manufacturing companies to provide actions put the young company at their mercy, and besides, Roy Weatherby wanted an action designed specifically to handle his growing line of magnum cartridges, including the large .378 and .460 Weatherby Magnums. The project proved to be a huge undertaking that took four years to bring from an idea into production.

When compared to other common American bolt-action rifles of the era, the Mark V was a radical departure and broke some of the old rules. For example it featured nine locking lugs and had a bolt-handle lift of 56 degrees, rather than the usual 90 degrees used on conventional two-lug bolt actions. (Today’s Mark V is advertised with a bolt-handle lift of 54 degrees.) Like many bolt guns designed after World War II, the receiver was round, making manufacturing easier with less machining steps. The bolt face was countersunk and featured a plunger ejector and an external rotating extractor. The bolt body was the same diameter as the locking lugs and the receiver ring void of raceways. The bolt was forged and carried eight flutes cut lengthwise in the body to help reduce binding.

To protect the shooter in the event of a ruptured primer or case, there was a gas-deflecting flange on the rear of the bolt assembly, and the bolt sleeve was enclosed to channel gas down through the rear of the action and away from the shooter. Any gases that might escape through the firing pin hole were diverted out three ports on the right side of the bolt body (at least on a right-handed gun) and away from the shooter. The two-position safety was quiet to operate, but more importantly, it locked the firing pin and disengaged the trigger sear.

The action was long enough and had a bolt face large enough to easily house even the largest Weatherby cartridges such as the .378 or .460 Weatherby Magnums, which was not possible for competing Remington and Winchester bolt guns. At any rate, it wasn’t long until advertisements appeared stating the Weatherby Mark V was the “world’s most powerful rifle . . . now with the world’s strongest action.” This naturally caused some concern at Remington, as it had been promoting its 721/722 rifles as such. I have no idea which action is actually stronger, but the Weatherby has withstood 200,000 CUP, more than is required for any of today’s cartridges and can certainly be considered ultra safe and brutally strong.

Early Weatherby Mark Vs were actually manufactured in South Gate, California, with probably less than 5,000 units made, but by 1959 J.P. Sauer & Sohn in West Germany began producing the rifles. Weatherby was happy with the German quality and was probably elated to have the worry of production off his shoulders. Then world economic conditions required Mark V production be moved to Japan where Howa Machinery began shipping guns in 1971 and German production was phased out in 1972.

Beginning in 1995 Weatherby again moved production from Japan back to the U.S., full-circle to where it started, but not in the original facility. Today’s version is CNC-machined and is absolutely the best Mark V ever built. The action has been continuously refined and improved to work smoother and more reliably than ever, culminating 45 years of production and hunting experiences. The trigger assembly is more positive and easier to adjust for weight of pull. The machining is definitely better than previous versions including the revered German guns - a statement that may cause dismay with Weatherby collectors.

Just as they have for many years, Weatherby guarantees its rifles to place three shots within 1 1/2 inches at 100 yards from a cold barrel. However, it has been my observation that guns built in the past few years will generally do better than this, as some out-of-the-box rifles have been observed grouping into 1 1/2 to 2 inches at 200 yards! Now let’s see how this custom shop rifle shoots.

A Burris 6-24x50mm Black Diamond scope with a 30mm tube and Ballistic Mil-Dot reticle was mounted on the new Weatherby. Having used the Black Diamond previously on rifles that dish out fast and heavy recoil, which can prove damaging to some scopes, I had confidence its tough-as-nails construction would allow it to stand up to the .30-378’s recoil. Besides having exceptionally clear and bright optics, the Black Diamond features a side focus parallax, a great idea and long overdue.

Previous models (and most other scope manufacturers) have the adjustable parallax ring located on the objective. In order to see the settings and adjust it, the rifle muzzle must be turned upward. This takes time and can be impractical under some field conditions, such as lying prone. The Burris side focus parallax can be adjusted quickly from almost any position using the left hand while the rifle is held to the shoulder and the muzzle pointed downrange.

The click adjustments move the crosshairs in 1/8-inch increments and worked perfectly throughout the shooting sessions. A Burris base and Signature rings were used, offering stress-free mounting. These won’t scratch or harm the scope in any way.

After cleaning the factory solvents/oils from the bore, the rifle was sighted for 200 yards, using Weatherby factory ammunition containing the 200-grain Nosler Partition. Often rifles of this nature will shoot almost as well at 200 yards as they do at 100 yards, due to bullet instability. In other words, the bullet is not fully stable at 100 yards but usually is at 200 or even 300 yards, and groups are almost as tight at the longer distances as they are at 100 yards. After the scope was adjusted and the barrel cooled, the first group, consisting of five shots, went into 1.3 inches.

The next load tried was the 180-grain Nosler Ballistic Tip, which managed to place five shots into 1.15 inches. This load placed three shots at 300 yards under 2 inches.

Before trying the last factory load with a 180-grain Barnes X-Bullet, the barrel was completely cleaned using Bore Tech Industries Copper Remover. While there are many good products of this type, this is the best one I have used, as it removes all copper in just a few minutes with minimal brushing. Three shots were then fired to foul the bore and the barrel allowed to cool once more. (Yes, it does heat up in a hurry.)

The first three shots landed inside an inch at 200 yards, while the last shot opened it up and was probably my fault. This Idaho summer has been unusually hot, and even in the shade, guns remain hot for long periods, so I fired another group without allowing the barrel to cool. The first three shots went into less than .5 inch while the fourth shot opened the group to 1.45 inches. The supply of factory fodder was depleted, which was unfortunate. I would like to have tried another group with this load, but from a cool barrel.

Velocities of the above factory loads can be seen in the accompanying table. I was amazed and maybe a bit surprised at the low extreme spreads that were observed with this cartridge in both factory loads and handloads. The 180-grain factory loads gave less than 25 fps extreme spreads for a five-shot string. Handloads containing Hodgdon H-1000 and Alliant Reloder 25 produced similar results.

Several handloads were assembled using 168-grain Barnes XLCs (coated X-Bullets) and the new 168-grain Triple-Shocks, as well as 180- and 200-grain Nosler Partitions and 180-grain Nosler Ballistic Tips. At .30-378 velocities, this latter bullet is so frangible that it is best as a varmint round, while the other bullets should have the integrity for taking big game.

Slow-burning propellants, such as Reloder 25, H-1000, IMR-7828 and AAC-8700, can equal or even exceed the velocities of the factory loads. These powders also gave reasonable extreme spread variations and good overall accuracy. My supply of H-870 and H-50BMG was running low, so these powders weren’t tried, but reports indicate they also produce excellent results in this big .30-caliber Weatherby.

Of the handloads tried, I was impressed with 107 grains of RL-25 with a 180-grain Nosler Partition, which produced 3,438 fps and grouped into 1.4 inches at 200 yards. Early indications are that it is grouping at just a shade over 2 inches at 300 yards. Another load that shot slightly better consisted of 109 grains of RL-25 with the 168-grain Barnes XLC for 3,505 fps and 1.1-inch, 200-yard groups. While H-1000 was only tried with the 180-grain Nosler Partition, it demonstrated low extreme spreads, and I’m sure that with further load development it will give excellent accuracy.

Handloads listed in the accompanying chart should be considered maximum, so start a few grains below these loads and work up carefully watching for signs of excess pressure. Incidentally, great savings can be recognized through handloading the .30-378, and high performance loads are not difficult to assemble.

Recoil with the Accubrake installed is mild-mannered but quick. Almost anyone should be able to fire this rifle without discomfort. Weatherby claims the Accubrake reduces recoil up to 53 percent. Out of curiosity, I fired a shot, removed the brake and installed the barrel thread cap then fired again. Recoil was noticeably increased, but by how much I have no idea. It is still a long way from being uncomfortable, as the weight of the rifle and scope certainly help tame it. (It should be pointed out that muzzle blast is significant with the Accubrake installed.) Another factor that makes this gun pleasant to shoot is the adjustable stock, which when set up specifically for the shooter, helps reduce felt recoil.

Obviously this latest Mark V and the .30-378 are specialized tools for long-range work - a job they do very well.