Weatherby Introduces The .338 WBY RPM Cartridge

Hrachya H
by Hrachya H
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Earlier this year, SAAMI accepted the 16th Weatherby cartridge, the .338 Weatherby Rebated Precision Magnum (.338 WBY RPM). Weatherby has now officially introduced the .338 WBY RPM cartridge and announced the availability of ammunition and rifles chambered in this new caliber.

Weatherby @ TFB:

“I am truly honored to be able to carry on my grandfather’s legacy of cartridge development and rifle building. The pairing of this new cartridge with our latest rifles reminds me of one of his old slogans that said “Tomorrow’s Rifles Today.” I only wish he could be here to see the innovation and development that our amazing Sheridan-based team is able to accomplish.”

Adam Weatherby

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The .338 WBY RPM is created by necking up the earlier released 6.5 WBY RPM cartridge. Both of these rebated-rim Weatherby cartridges have the same rim (.473″) and base (.500″) diameters as the .284 Winchester, however, they have longer cases (2.55″/2.57″ vs 2.17″) and operate at higher SAAMI Maximum Average Pressures (65K vs 56K psi) than the .284 Win. Below you can find the SAAMI drawing of the .338 WBY RPM cartridge and the full SAAMI data sheet can be downloaded HERE.

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The dimensions of this cartridge are tailored to make it the possibility to chamber it in the Weatherby Mark V Standard 6-lug action which is much more compact and lighter than the Mark V Magnum 9-lug action. According to Weatherby, this cartridge and action combination offers magnum performance in a lightweight package and provides the ultimate lightweight backcountry setup with heavy-hitting magnum performance. The size difference between these two Weatherby actions is demonstrated in the picture below.

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The .338 WBY RPM ammunition currently offered by Weatherby is loaded with the following .338 caliber bullets: 185gr Barnes TTSX, 225gr Barnes TTSX, 225gr Hornady Interlock and 225gr Nosler Accubond. The box of 20 rounds of the .338 WBY RPM 225gr Hornady Interlock load can be found on Weatherby’s website at an MSRP of $79. The other three options cost $109.

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Here is the muzzle velocity data of all four loads, for 24″ and 18″ barrel lengths. For more detailed information on the velocity, energy and drop of these bullets at different ranges, visit the Weatherby website page dedicated to the .338 WBY RPM cartridge.

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Currently, two Weatherby rifles are chambered in the new .338 WBY RPM cartridge – Mark V Backcountry 2.0 Ti 18″ and Mark V Backcountry 2.0 Carbon 20″. These rifles have MSRPs of $3,449 and $3,299 respectively.

Weatherby Mark V Backcountry 2.0 Ti 18
Weatherby Mark V Backcountry 2.0 Carbon 20

Lastly, here is a video published by Weatherby telling about the development and field testing of the new cartridge.

Pictures by Weatherby ( and SAAMI (

Hrachya H
Hrachya H

Managing Editor Being a lifelong firearms enthusiast, Hrachya always enjoys studying the history and design of guns and ammunition. Should you need to contact him, feel free to shoot him a message at

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