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.338-378 Weatherby Magnum

The .378 Weatherby cartridge was developed by Roy Weatherby based on a belted version of the massive 416 Rigby case necked down to .375”. This was introduced as a Weatherby proprietary cartridge in 1953. During the 1960’s, U.S gun writer Elmer Keith and Bob Thompson created the wildcat .338-378 by shortening the .378 case slightly and necking it down to .338”. In 1998, due to continued interest in the wildcat, Weatherby adopted the .338-378 Weatherby based on the full length .378 case as a factory cartridge.
Prior to Keith and Thompson’s development of the .338-378 wildcat, Keith was promoting the virtues of high velocity medium bores firing heavy, long for caliber bullets for use on large bodied (Elk upwards) North American game. In 1945 with the help of fellow enthusiasts, Keith developed the wildcat .333 OKH based on the .30-06 case necked up to take .333” British projectiles. This cartridge was in Keith’s opinion adequate for close to moderate range hunting but not as effective as a higher velocity magnum. To this end Keith pursued the creation of the wildcat .334 OKH, developed two years later based on the larger .375 H&H case. 
In 1958 Winchester re-established .338” caliber projectiles with the introduction of the .338 Winchester Magnum. The standardizing of the .338” caliber opened the way for Weatherby to develop the more powerful .340 Weatherby in 1962.  Although the .340 duplicated the performance of Keith’s wildcat magnum, it would appear that the availability of .338” caliber projectiles inspired Keith to completely start over again with experimentation.
Seeking performance in the extreme, Keith with the help of Bob Thompson developed a new wildcat based on the .378 Weatherby case necked down to .338”. Various sources state that Thompson was eager to use the full length of the .378 case however Keith insisted and  convinced his cohort to shorten the .378 case by a quarter inch to better suit the powders of the day. Even though it was short, the wide case would ensure high velocities.
Keith promoted the resulting .338-378 KT wildcat as being ideal for taking large game at longer ranges and although its popularity was limited, the .338-378 KT gained a steady following. This cartridge is notable as having a part in the development of the .338 Lapua Magnum (see .338 Lapua). In 1998 Weatherby standardized the full length .378 Weatherby case (2.908”/73.86mm) necked down to .338”, naming it the .338-378 Weatherby. The .338-378 Weatherby is currently the most powerful .338” factory cartridge. In a similar manner to the .338 Lapua, the .338-378 employs a full .579” diameter case rim rather than a standard magnum rim diameter of .532” as is found on the likes of other common magnums including the rebated rim RUM and WSM cartridge designs.
Compared to the .338 Lapua and .338 Edge, the .338-378 shows a velocity advantage of around 100fps. In practical terms, this equates to an increased effective range of around 50 yards. Nevertheless, from a purely historical data based perspective the Weatherby remains king in the velocity stakes.
The .338-378 Weatherby currently maintains a modest following for hunting big game at long ranges. The Lapua and its tactical appeal have certainly stole some of its thunder but there are still a good many hunters who loyally stand by the Weatherby.


To obtain high velocities without excessive pressure the .338-378 Weatherby, like its kin, utilizes a good deal of freebore. This can be quite acceptable however it can also make a rifle finicky when it comes to the extreme accuracy required for long range work. There is also a big difference between those who say they shoot long as opposed to those who actually do shoot long and are able to obtain one shot kills in a clean and consistent manner. It must also be understood that in former years long range hunting meant being able to reach across a valley to 400 yards with 500 yards being a long shot. Now with modern optics, an Elk can be cleanly taken with a .30-06 and 208 grain bullet at these ranges without fuss. By the same token, our expectations of the .338’s are now higher with expectations of precision accuracy past 1000 yards. This places considerable demands upon the rifle and its cartridge design. Fortunately Weatherby have tried their best to keep up with the times by producing such rifles as the Weatherby Threat Response rifle. Although this is a heavier tactical type configuration, the rifle boasts straighter lines than Weatherby’s traditional rifles. The cartridge design may not have changed but there are a good many shooters who have put the time and effort into obtaining extreme accuracy with rifles of this ilk, fully utilizing the .338-378 Weatherby.
With regards to killing potential, the .338-378 certainly has prowess for hunting large bodied game. I still do not consider this overkill for lighter animals. I believe much of this is determined by bullet choice as opposed to the physical size of the cartridge and our visual perception. Also, I would far rather see readers over gunned than under gunned – providing they are comfortable with the cartridge and can shoot straight. Nevertheless, the .338-378 is more of a specialized cartridge. Brass is expensive, it consumes copious amounts of powder and it takes dedication in order to extract its full potential.
Without attention to detail the .338-378 (and .338 kin) can be disappointing. For example, there are those who still believe in this age of information that power can be used as a substitute for accuracy. Such bragging still occurs at camp fires and rifle ranges. The use of power in this manner may certainly work at close ranges where velocity is high, but once we reach out, stout bullets may pass through lean game without fully expanding and fully imparting energy, resulting in slow kills. In contrast to this, there is the potential to use a bullet that is altogether too soft for use on large heavy bodied game at close ranges. Yet again, bullet choice and shot placement are proven to be equally important. This is a subject that gets under the skin of many hunting guides.
If you really want great power, then it is perhaps best that you know how to use it.

Factory ammunition

Weatherby currently list two loads for the .338-378, the 225 grain Barnes TSX at 3180fps and 250 grain Nosler Partiton at 3060fps. Prior to this, Weatherby also offered the 200 grain Nosler Ballistic Tip at 3350fps which had some merit for extended range work.
The 225 grain TSX has both the speed and construction to deliver fast killing of game weighing between 90 and 320kg (198-700lb) out to a range of around 200 yards producing clean kills out to around 400 yards (2200fps). This bullet is a clean killer of heavy game up to 1400lb and is the most reliable penetrator of the Weatherby .338-378 loadings. This bullet is not ideally suited to long range work whether as a TXS or any potential Tipped TSX offerings. The construction of this bullet is simply too stout to produce wide wounding on large game at low velocities, especially where wind drift is a concern.
Although the 250 grain Nosler Partition is a premium bullet it should not be expected to hold together when encountering heavy bone at close range. In the .338-378 and from a realistic MV of 3000fps, the 250 grain Partition produces fast kills (hydrostatic shock) on game weighing between 90 and 320kg out to a range of around 200 yards, produces very broad wounding down to 2200fps (420 yards) before running out of steam at 1800fps or 675 yards. The Partition does suffer a good deal of wind drift and shot placement does become critical once the 2200fps barrier is breached, therefore 675 yards should be considered the outer limit for this bullet.
The 250 grain Partition has been used on Moose with great success by many hunters shooting .338-378 Weatherby rifles  however it must be said that this bullet can be prone to tumbling and the loss of its core if it is pushed too hard into heavy round ball joints. Partition is more able to handle larger game such as Moose in a reliable manner at ranges beyond 300 yards.
Nosler currently offer two factory loads for the .338-378 featuring the 225 grain Accubond at 3250fps along with a 250 grain Partition load, duplicating Weatherby’s offering.
In the .338 Wincester Magnum text I have stated that the 225 grain Accubond can at times prove to be neither fish nor fowl. But with extra velocity, this bullet can prove more useful. In the .33-378 and when used at normal hunting ranges this is a very fast expanding bullet, its core bonding serving to help prevent early jacket core separation, though the bullet may still come apart after initial penetration. And while this may sound somewhat unreliable, such performance can be put to great use. This is best suited to game weighing between 90 and 320kg (200-700lb) out to ranges of around 600 yards, tapering off in performance thereafter. In other words, it can make for a great mountain load. On heavier animals weighing up to 450kg (1000lb), the Accubond is too frangible to be called truly reliable at very close ranges however once it passes below 3000fps, it can be used to take rear lung / meat saver shots. This is quite a useful load for those who hunt Elk.

Hand loading

 Handloads for the .338-378 typically duplicate but do not normally exceed factory figures. Using ultra slow burning powders in the IMR 7828 and ADI 2217 range the .338-378 produces velocities from a 26” barrel of 3500fps with 180 grain bullets, 3300fps with 200 grain bullets, 3250fps with 210 grain bullets, 3200fps with 225 grain bullets, 3050fps using 250 grain bullets, 2850fps with the 285gr A-MAX / ELD and 2750 to 2800fps with heavy 300 grain bullets.  Some reloaders record velocities up to 100fps higher than these however figures listed here are realistic averages.
Hornady offer an extremely useful range of .338 hunting projectiles. For light framed game, the 200 grain SST is an excellent performer. Driven at 3300fps, this bullet is spectacular out to ranges of around 400 yards. Following this, the 200 grain SST is capable of producing broad wounding out to ranges of around 900 yards (1600fps). That said, once the 200 grain SST breaches 1800fps, it becomes ever more reliant on either body weight resistance or shot placement to initiate expansion. There can therefore be quite a difference in performance when for example we compare results on Fallow deer versus Red deer or White Tail deer versus Mule deer at such ranges. If game are truly lean and where there is a potential for high wind drift, I do not recommend that this bullet be pushed past 750 yards - still a very long way.
The 225 grain SST can be used on both lean and larger bodied animals at normal hunting ranges but if using the .338-378 at long ranges, the 225 grain SST should be relegated to game weighing between 90 and 320kg (200-700lb). This projectile is another spectacular killer out to between 900 and 950 yards but again, care must taken with game body weights.
The Hornady Interbond (and Nosler Accubond) can be used to effect fast killing at ordinary ranges on Red deer to Elk sized game. This is a rather stout bullet but when driven at very high velocities it creates very wide wounding out to moderate ranges and can be highly useful. The Interbond should not be considered ideal for larger game at close ranges but can be used for meat saver shots once its initial velocity is shed. There are a small number of hunters who use this bullet for extended range work on large bodied deer species. At these longer ranges however the Interbond is very much reliant on impact velocities of 2200fps and higher for maximum wounding. There are far better bullets for this role such as the SST and A-MAX / ELD.
The 285 grain Hornady A-MAX (now called ELD) is one of the best things to have happened to the .338 bore and especially the .338-378. The Weatherby helps ensure that velocity is kept high which in turn allows this bullet to produce consistent performance (width of wounding) on wide range of game out to ranges exceeding 1200 yards. The only bullets which bear close comparison for long range work are those produced by Rocky Mountain.
Rocky Mountain ULD bullets are available in the weights 225, 250, 275, 300 and 325 grains. These are true long range killers and as with all projectiles should be selected based on game body weights as opposed to for example, selecting the lightest bullet because it is travelling the fastest or the heaviest bullet because it will out penetrate others. Common sense must always be applied. These bullets can make the Weatherby (also the 340, Lapua and Edge) usable to ranges beyond 1000 yards, exceeding 1500 yards in expert hands. 
Nosler’s Partition bullet design can be useful in the .338-378 however these do have limits. Each can be used with great success on Elk sized game. On heavier game, there is a risk of jacket / rear core separation as a result of high impact velocities should these encounter round ball joints (which you should be aiming towards). Heat is also a factor, something I do not discuss a great deal but is a part of the equation. When driven fast from the .338-378, heat from both bore friction and air resistance can soften the lead core of a bullet and make it that much harder for a bullet to hold together on impact. I have said this numerous times but high sectional densities are the key to ensuring deep penetration when using the Partition. This and being realistic as to what to expect from the bullet versus game body weights and shapes. Unfortunately in the .338 bore, Nosler simply do not have a Partition that is long and heavy enough to meet this criteria - at least not at .338-378 speeds. Nosler’s heaviest bullet, the 250 grain Partition is perfect for Elk hunting and can do equally well on Moose at close ranges versus .338-378 WBY speeds. But if we change the shape of a Moose to that of a bovine, even though the weights may be identical, the bovine displays a shorter yet much wider body cross section which can include densely packed muscle and bone and this can present too much of a challenge for our Partition bullet. The Accubond fairs far worse under similar circumstances.  
When hunting heavy game there is nothing quite like getting in close with a heavy bullet driven fast in order to obtain a horrendous crash as a result of a fast on the spot kill. And although there are not a great many heavy and truly tough bullet offerings for use at .338-378 velocity, the 300 grain Woodleigh Weldcore is an emphatic killer. The .338 bore may not fire bullets with the same frontal area as the .375 and larger bores, however a bullet like the 300 grain Weldcore can be driven at relatively high velocity in the .338-378 and has great sectional density on its side. Hunters are reminded to place shots into the forwards chest cavity of large bodied game and aim to break major bone, especially with this smaller bore diameter.  
Swift also make a useful large game bullet - the 275 grain A-Frame projectile. This is not quite as heavy as the Woodleigh but is a good choice, especially in countries that do not stock Woodleigh products. The rules here are the same, drive it fast, get in close and aim small.
For those wanting maximum penetration, Barnes offer TSX bullets up to 285 grains in weight in TSX, TTXS and LRX configurations. These do their best work at high impact velocities this also includes the LRX. The LRX may be able to expand at low impact velocities but this does not mean to say that it can actually impart maximum energy at low velocities. All of the Barnes bullets are capable of rendering very wide wound channels but again, it pays to keep shots well forwards.

Closing comments

The .338-378 excels as a long range big game cartridge. Unfortunately, like other medium and large bore cartridges, a seemingly large bullet and high velocity do not on their own produce deep penetration or wide wounding as one might expect when observing such a cartridge in the palm of one’s hand. Bullet construction must be matched to the job at hand in order to extract the full potential from this power house. It is no good having the fastest car if you don’t have the right tires.
The same can be said of rifle bedding, trigger settings and shooter technique. It is no good having the fastest car if you do not know how to drive it!
Suggested loads: .338-378 Weatherby Barrel length: 26-28”
No ID   Sectional density Ballistic coefficient Observed  MV Fps ME
1 HL Hornady SF 225gr SST/IB/AB .281 .515 3200 5115
2 HL 250gr Rocky Mountain .281 .846 3050 5163
3 HL 285gr A-MAX / ELD .356 .820 2825 5139
4 HL 300gr Woodleigh RN .416 .332 2800 5221
Suggested sight settings and bullet paths         
1 Yards 100 175 302 345 375 400 425
  Bt. path +3 +4 0 -3 -5.8 -8.5 -11.5
2 Yards 100 150 296 340 375 400 425
  Bt. path +3 +3.7 0 -3 -6.2 -8.8 -11.7
3 Yards 100 150 270 312 350 375 400
  Bt. path +3 +3.7 0 -3 -6.6 -9.4 -12.6
4 Yards 50 100 210 250 300    
  Bt. path +.8 +2 0 -2.6 -7.3    
No At yards 10mphXwind Velocity Ft-lb’s
1 300 5 2644 3491
2 300 3 2714 4089
3 300 3.6 2519 4015
4 100 .8 2582 4441
 338 378 Weatherby final
.338-378 WBY Imperial Metric 
A .579 13.51
B .582 13
C R.151  
D .561 12.5
E .370 9.27
F 2.345 58.36
G .353 8.97
H 2.913 74
Max Case 2.913 74
Trim length 2.903 73.7
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We are a small, family run business, based out of Taranaki, New Zealand, who specialize in cartridge research and testing, and rifle accurizing.