The emergence of the new 338 WBY RPM can be likened to the formation of new stars. Gravity pulls in matter that is assimilated into the physical dimensions and fused to form a stellar nebula. From that a new star is formed.
The 338 WBY RPM is quite evolutionary. By the way, “RPM” stands for “Rebated Precision Magnum.” The 338 WBY RPM is the second in what we might call the “RPM Universe” and offers a unique avenue to reloading opportunities for the shooter and big-game hunter.
First, a little background. The name Weatherby is well known in shooting orbits, but the 338 WBY RPM is a huge departure from the standard Weatherby paradigm. Cartridges from the firm that Roy Weatherby started in 1945 are based on a simple but profound concept: High velocity kills. (Yes, I know the proper term is “speed,” since “velocity,” as Mark Denny has recently pointed out, describes how fast and in what direction an object is going. “Speed” just tells us how fast it’s going. But I’ll use “velocity” here.)
Roy was obsessed with velocity and designed his cartridges to achieve higher velocities than “standard” cartridges of a given caliber. To do this, for most of them, he used a cut-off and blown-out belted magnum case, necked to the caliber under development. This increased the powder capacity. In addition, the barrels of rifles chambered for Weatherby cartridges had long throats, or lots of “freebore.” This gave the bullets a running start at the rifling, and stiff powder charges gave maximum velocities. Another characteristic of the Weatherby case was the double-radius shoulder. Weatherby’s early PR literature implied this produced better ballistics, but rumor has it that it may have been a marketing ploy.
A Star is Born
The 6.5 WBY RPM was introduced in 2019. It is Weatherby’s 16th cartridge. (The first was the 270 Weatherby Magnum, dating from 1945.) It is remarkable in that the WBY RPM case deviates considerably from the usual Weatherby case. The WBY RPM doesn’t have a belt or a double-radius shoulder. What’s more, it has a rebated rim! None of this was for show; rather, it was for “go.” The rim is the same diameter as the 30-06, and many other cartridges, but the case body is about the same diameter as a belted magnum, starting at 0.500 inch and tapering slightly to the 35-degree shoulder. Also, WBY RPM rifle chambers do not have the typical Weatherby freebore. Furthermore, the overall cartridge length is short enough to fit in standard-length actions. These tweaks resulted in stellar ballistics from the new round.
The Weatherby folks thought a big-bore cartridge that would fit into the “standard” length actions, like the 6.5 WBY RPM, would be a welcome addition to what the late Warren Page called the “cartridge picket fence.” Such a round would propel large-diameter, heavy bullets at velocities that nibbled at the velocities of existing 338 Magnums but could be housed in a trimmer and lighter rifle. Thus, the concept of the new 338 WBY RPM was born.
The 338 WBY RPM is merely the 6.5 WBY RPM case necked up to .338 caliber. The new round was approved by S.A.A.M.I. in January 2022. Its maximum cartridge overall length is 3.25 inches, only 0.090 inch longer than the 30-06, and recall that its rim is the same diameter as the 30-06. The neck length of the 338 WBY RPM is 0.314 inch, or .93 caliber, which is very close to the one-caliber principle that’s considered optimal.
The good folks at Weatherby loaned me a brand new Mark V Backcountry 2.0 rifle with an 18-inch barrel for this special report. The 338 WBY RPM is also offered in the Backcountry Ti (also with an 18-inch barrel) and the Backcountry 2.0 Carbon and Ti Carbon with 20-inch barrels. The Accumark and Carbonmark models have 24-inch barrels. All have 1:10 twists. My Mark V Backcountry test rifle weighs five pounds, five ounces, without a scope.
Speaking of scopes, I used a new Sightron Series G2 S1 4-12X 40mm scope. This excellent glass has 0.25-minute clicks and crystal-clear optics. It weighs only 14.1 ounces. The weight of the scoped rifle is 6.19 pounds, which makes it great for toting over western hunting country.
The size and shape of the 338 WBY RPM round’s case is just right to utilize many propellants, and the clever folks at Hodgdon’s sophisticated ballistics labs were quick to see the potential of this new round. They have carefully developed dozens of pressure-tested loads with bullets ranging from 185 to 265 grains. The data for 130 pressure-tested loads are shown elsewhere in this manual, and they also are available online at the Hodgdon Data Center. I’ve compiled a list of powders suitable for the 338 WBY RPM, and it is shown in the accompanying chart. I can honestly tell you the 338 WBY RPM is not picky about the powder it’s fed, as many work very well with a variety of bullets. The maximum average pressure (MAP) set by S.A.A.M.I. for the cartridge is 65,000 psi, and none of Hodgdon’s loads exceed that limit.
Cartridge Comparisons
As you can see from the cartridge comparison chart, the 338 WBY RPM virtually matches the classic 338 Winchester Magnum in performance, and Hodgdon’s load data confirms that. This places it in good company indeed, as the 338 Win. Mag. is the quintessential western cartridge for hunting elk and mule deer. The neat thing about the 338 WBY RPM is it can be housed in a rifle that is lighter and trimmer.
Let’s look at the two rounds in a side-by-side comparison of maximum velocities with several bullet weights. I’ll point out that the 338 Win. Mag.’s MAP is 64,000 psi, which is a tad lower than that of the 338 WBY RPM. But this is as close to an “apples-to-apples” comparison as we can get, as all these data were obtained in Hodgdon’s minimum S.A.A.M.I.-spec pressure barrels that were 24 inches long and had 1:10 twists.
With the Barnes 185-grain bullet, the 338 WBY RPM achieved 3,150 fps with 73.5 grains of StaBALL 6.5. Note that the pressure for this load is 2,000 psi below the round’s MAP. The 338 Win. Mag. went 3,086 fps with 72.3 grains of the same powder.
Moving to 200-grain bullets, the 338 WBY RPM launched the Federal Fusion bullet at 3,065 fps with 73.5 grains of StaBALL 6.5. The 338 Win. Mag. pushed the Speer 200-grain Softpoint to 2,979 fps with 74.0 grains of H4350.
The 250-grain bullet is a favorite of .338-caliber elk hunters, and the 338 WBY RPM fared well here, too. With the Sierra 250-grain Spitzer Boattail, a charge of 71.3 grains of Hunter produced 2,738 fps. The 338 Win. Mag. hit 2,889 fps with 72.5 grains of IMR 4831.
There are even data for 265-grain hunting bullets. The 338 WBY RPM favored a charge of 63.9 grains of StaBALL 6.5 with the sleek Barnes 265-grain LRX. Velocity was 2,655 fps. This bullet in the 338 Win. Mag. with 63.0 grains of Hybrid 100V produced 2,496 fps. And the list goes on.
The larger cartridges outclass the 338 WBY RPM, but at the expense of carrying a heavier rifle and burning much more powder that produces more muzzle blast and recoil. The 338 Lapua, for example, takes 39 percent more powder than the 338 WBY RPM to gain just 6.6 percent more velocity.
Loads for the 338 WBY RPM
The 338 WBY RPM is so new that reloading dies were not available at this writing. However, I noticed the 33 Nosler and the 338 WBY RPM cases are similar in dimensions. Since I already had 33 Nosler dies on hand, I gave them a try and discovered that I could seat bullets in charged 338 WBY RPM cases and “neck size” fired cases with the sizing die. So all my test loads were prepared with this setup, and it worked perfectly. I used these dies in my Redding T-7 turret press, and I primed cases with the Hornady Hand Held Priming Tool. All charges were weighed on an RCBS M-1500 electronic scale. Case lengths were kept to 2.540 inches, 0.010 inch under maximum case length.
Test loads were fired from a Lead Sled DFT inside my shooting building over my Oehler Model 35P chronograph at targets placed at 100 yards. Due to the paucity of components, I fired one three-shot group with each load combination. Nevertheless, the group sizes of the loads I tested (listed in the 338 WBY RPM load data chart that accompanies this report) were pretty darn good.
I relied on Hodgdon’s data as a guide in my tests, and none of my loads exceed that pressure-tested maxima. In fact, based on the appearance of the primer and case condition, it looked to me like the loads I used were about “working max” for this rifle.
A number of loads showed considerable promise. As expected, Sta- BALL 6.5 turned in the highest velocities overall, and groups were rewarding. If there was a trend in the shooting results, I would say that it was that bullets of 250 grains or less worked best with this terrific new powder. There just wasn’t enough powder space to give the long 265- and 275-grain bullets enough velocity to make use of their high ballistic coefficients downrange. You know the old saying: “You can’t get 10 pounds of you know what in a five-pound sack.”
But this would be no hindrance, as several Hodgdon powders gave the 200-, 210-, and 225-grain bullets velocities that produce flat trajectories and plenty of game-stopping power across a canyon.
Overall, the performance of many propellants was more than adequate with each of the test bullets. I calculated the coefficient of variation (COV) for the test loads. COV is the standard deviation expressed as a percentage of the average velocity, and a value of around 1.0 to 1.5 percent is excellent. The COVs of all test loads were less than 1 percent, but that still indicates very good ballistic uniformity.
Several powder and bullet combinations produced eye-opening results. The Barnes 160-grain TTSX FB over 72.0 grains of StaBALL 6.5 gave a velocity of 2,785 fps, and its excellent group measured just 0.47 inch. Another tight cluster was turned in by the Hornady 200-grain Spirepoint with 69.2 grains of Hybrid 100V at 2,663 fps. These three bullet holes were only 0.40 inch apart. The 225-grain SST from Hornady liked a charge of 57.8 grains of Varget. Velocity was a modest 2,417 fps, but the group was only 0.55 inch. A charge of 63.5 grains of StaBALL 6.5 and the Hornady 250-grain Spirepoint teamed up for a velocity of 2,375 fps and a group size of 0.67 inch.
I just had to try the 265- and 275-grain bullets, and while the group sizes were perfectly acceptable, the velocities weren’t as impressive. The Barnes TTSX BT over 57.8 grains of Big Game produced a 0.85-inch group at a velocity of 2,264 fps. The 275-grain Swift A-Frame with 62.0 grains of StaBALL 6.5 clocked 2,351 fps, and the group was a delightful 0.65 inch.
I cannot leave the “results” section without reporting the recoil of the test loads. A huge component of perceived recoil is the velocity of the gun when it’s fired. As Major Sir Gerald Burrard reported in 1961, a recoil velocity over 16 fps is “too great to be overcome.” How right he was!
The average recoil velocity of the test loads was 19.8 fps. The average recoil energy was 37.8 ft-lbs, and five loads were over 40 ft-lbs. The combination of these two factors makes the recoil of the 338 WBY RPM a memorable experience.
If I were to pick a “favorite” powder for the 338 WBY RPM, it would be StaBALL 6.5. This powder is truly revolutionary, in that it was the first Ball powder that combines temperature insensitivity, precise metering, low standard deviations for velocity and pressure, and higher velocities than other powders in its burn-rate class, sometimes as much as 200 fps, depending on the cartridge.
However, there are so many good powders for the 338 WBY RPM that the handloader is sure to find a fine load with several bullets. For example, powders in the burning range from Varget through StaBALL 6.5 do well, as does the classic IMR 4064. For the 250- and 265-grain bullets, H1000 and IMR 7828 take top honors.
The number of loads a lab can develop in a certain time frame is limited, so it is not surprising that data for some powders that would be considered “usual suspects” are missing from the current data set.
My guess is that CFE 223 would work well in the 338 WBY RPM, as it is in the right burning range. My experience with CFE 223 and Sta-BALL 6.5 is nothing short of remarkable unless you like high standard deviations and cleaning rifle bores. The 338 WBY RPM is a fertile field that needs to be plowed, and I’m sure it will be.
Hodgdon’s data show the versatility of its powders in the new 338 WBY RPM. Things just keep getting better and better for us handloaders, and I predict that the 338 WBY RPM will become a “massive star” indeed.