.30-378 Weatherby Magnum

Weatherby's fire-snorting .30 caliber got its start with the U.S. military.

Weatherby's .30-378 is the .378 Weatherby Magnum necked down to .30 caliber and can drive a 180-grain bullet in excess of 3,400 fps.


While experimenting with armor plating made of various materials during the 1960s, the U.S. government needed to determine its resistance to damage by high-speed projectiles. So designers turned to Roy Weatherby, who had earned his title of "King of High Velocity" by introducing a series of rifle cartridges that left the competition choking in the dust.

Soon after receiving a contract from the government, Roy simply necked down his .378 magnum case to .30 caliber, seated special 30-grain bullets made by Vernon Speer atop heavy charges of the slowest-burning powder available and exceeded a muzzle velocity of 5,000 fps.


Sometime later, the availability of an extremely dense propellant with an even slower burn rate increased velocity to just beyond 6,000 fps. And so it happened that Uncle Sam got its ultra-velocity cartridge and Roy Weatherby picked up a bit of extra money. But the .30-378 magnum story does not end there.


If not for interest in the cartridge by members of the Original 1,000-Yard Benchrest Club in Pennsylvania during the 1960s, the .30-378 magnum would most likely have faded away. It became quite popular among a number of shooters in that club.

One of them was Earl Chronister, who held the world record for the smallest 10-shot group fired in 1,000-yard competition. His first nine 250-grain Sierra MatchKing bullets fired from a rifle in .30-378 magnum snuggled into a group measuring 3.125 inches, and even though a sudden shift in wind caught him off guard on the 10th squeeze of the trigger, the group still measured an incredible 4.375 inches. Even though that record has since been beaten, it was quite an accomplishment in its day.

The next important chapter in the life of the .30-378 magnum came later. During a conversation with Ed Weatherby in late 1991, I urged him to start chambering his rifles for the big cartridge and offer loaded ammunition.

The .30-378 magnum was introduced in the Mark V rifle in 1996, and shortly thereafter, I was asked for chamber dimensions and load data that I had worked up--based on a rifle in that chambering I'd built the year before--and the information was forwarded on to Norma so the Swedish firm could begin development of factory ammunition. (Norma has been loading Weatherby ammo since the 1950s.)

Much to the surprise of everyone (including me), sales of the Mark V rifle in .30-378 magnum far exceeded the projections of Weatherby and his staff. Overnight it became the best-selling cartridge in that rifle, a position it held onto until 2008 when it was nudged into a close second place by the .257 Weatherby Magnum.

Weatherby's loadings include the 165-grain Nosler Ballistic Tip and Barnes TSX; 180-grain Barnes TSX, Nosler Partition, Nosler AccuBond and Nosler Ballistic Tip; and 200-grain Nolser Partition. Velocity ratings for those three bullet weights are 3,500, 3,420 and 3,160 fps, respectively.

As is almost always the case with other cartridges, those who develop data for the reloading manuals tend to disagree on just how fast the .30-378 Weatherby Magnum really is. Some show the cartridge topping out with a 180-grain bullet at 3,300 fps while others give it another 100 fps or so.

Through the years I have worked with four different rifles in this cartridge, and only one of them was incapable of exceeding 3,400 fps--and that one had slightly undersized bore and groove diameters.

One thing is certain: The maximum velocity potential of the cartridge will be reached only by the use of the slowest-burning powders available. Top candidates on my loading bench are Reloder 25, H50BMG, US869 and Retumbo. If I had to pick just one for use with all bullet weights, it would be Retumbo.

It also takes a magnum primer to light the fire in this cartridge. While developing his .378 magnum back in the 1950s, Roy Weatherby was experiencing ignition problems with primers then available, so he went to Federal, which came up with a solution in the form of the 215 primer.

It went on to become standard in Weatherby ammunition, not only in the .378 Weatherby Magnum, but later in his .460, .416 and .30-378 magnums as well. And since the 215 primer is available from Federal as a reloading component, it is the logical choice for handloads as well.

It could be that other magnum primers such as the Remington 91⁄2M, CCI 250 and Winchester WLRM would work equally well, but the Federal 215 does such a good job in its role I have never gotten around to trying anything else in the big Weatherby cartridges.

Like any cartridge that consumes huge charges of powder for its bore size, the .30-378 magnum will burn out a barrel in fewer rounds than will a much smaller cartridge. But barrel life with this cartridge can be longer than some would have us believe.

Back in 1996 I spoke with Chronister about his accomplishments in 1,000-yard competition and asked him how often he replaced the barrel on his rifle. He told me every 2,000 rounds. That's not a lot of shooting for a varmint shooter, but it is more rounds than some hunters fire in a rifle during a lifetime.

The secret to enjoying acceptable accuracy life from the .30-378 magnum boils down to nothing more than using a bit of common sense: Keep rapid-fire shooting to a minimum, allow the barrel to cool down between groups and keep bullet jacket fouling under control through proper bore cleaning.

Abide by those three rules and you can live happily ever after with Roy Weatherby's biggest .30 caliber cartridge.

.30-378 Weatherby Magnum Load Guide
Bullet Bullet Weight (gr.) Powder Type Powder Charge (gr.) Muzzle Velocity (fps)
Barnes TSX 130 RL-25 116.0 3,812
Barnes TSX 150 RL-25 111.0 3,533
Barnes TSX 165 US869 124.0 3,511
Swift A-Frame 180 Retumbo 113.0 3,448
Swift A-Frame 200 Retumbo 105.0 3,225
Nosler Partition 220 H50BMG 118.0 3,110
Barnes TSX 165 Factory Load N/A 3,407
Nosler AccuBond 180 Factory Load N/A 3,439
Nosler Partition 200 Factory Load N/A 3,108
Notes: Powder charges are maximum and should be reduced by 10 percent for starting loads. Velocity is an average of five or more rounds clocked 12 feet from the muzzle of a Weatherby Mark V with 26-inch barrel. Weatherby cases and Federal 215 primers were used in all loads.

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