Early development
Ever since Roy Weatherby began manufacturing rifles he had to rely on a third party to provide the actions for his rifles. Beginning in 1949 Weatherby began building his rifles around the FN Belgian Mauser action. In 1955 Schultz & Larson actions were added to the mix while adding the Mathieu left handed action to his rifle action line up. With the addition of the large capacity .378 Weatherby Magnum a new action type was warranted so the Brevex Magnum Mauser action was added. A little later the FN Sako Mauser actions were added to action types used by Weatherby to build his rifles. Roy Weatherby had been disappointed by the length of time that was taken discussing and negotiating before coming to an agreement with his European manufacturing partners. With five European companies supplying actions, the frequent trips made to Europe left him little time to run the day-to-day business operations in South Gate, California. Roy Weatherby had found the Mauser-type wanting due to the case head not being completely enclosed and supported within the breech. While these action types could easily withstand a high pressure of about 70,000 C.U.P, he felt that one which would have the capability of handling 200,000 C.U.P. would fare better with the Weatherby line of cartridges. He had been aware that many handloaders were overloading their cartridges seeking higher performance, which was resulting in blown primers and ruptured case heads. This in turn would lead to hot gases making their way through the bolt and into the shooter's face and eyes and causing injuries to his customers. Weatherby's requirements included a bolt face which was countersunk into the bolt which would enclose the cartridge's case head while the bolt head was enclosed in a counterbored barrel breach, which in turn would prevent the case head from rupturing. If a rupture should occur, the bolt would have holes through which hot gases could be vented safely towards the side and away from the shooter's face. Further as an extra safety measure he required an enclosed or shrouded bolt sleeve that would prevent the hot gases from making their way through the bolt mechanism to the shooter's face. To Weatherby, the safety and strength of the design were paramount. The Weatherby Mark V action uses an interrupted screw breech as used on many artillery pieces. The nine lugs are actually sections of a screw thread. The bolt cams forward slightly as the bolt is closed following the pitch angle of the screw thread. Weatherby designed the action type with nine locking lugs arranged in three rows. It was determined early on that these lugs would not protrude outside the bolt but would be of the same diameter as the bolt body itself. Such a design would be conducive to a smooth fluid movement of the bolt through its feeding and extraction cycles. In spring 1955 Weatherby demonstrated his new action at the annual NRA meetings in Washington, D.C. The action type was shown to Burt Munhall of H.P. White Ballistics Laboratory and to General Julian Hatcher to elicit their opinion of the design. Both were impressed with the design and encouraged Weatherby to commercialize the action as soon as possible. At this time Weatherby had been looking for someone to provide the technical design, drawings and specifications for the design of his new action. General Hatcher suggested that John Garand (of the M-1 Garand rifle fame) would be suitable for the job. However, when contacted Garand expressed disinterest in the project as he was enjoying his retirement. Walter Howe, the editor of the ''Proof testing of the Mark V action
Weatherby had intended that the new action would be the safest and strongest bolt action available. The rifle was marketed as "The World's Strongest Bolt Action". The Mark V action has been tested to be able to contain up to 200,000 CUP (Copper Units of Pressure). The testing of the rifle was conducted on a production rifle chambered for the .300 Weatherby Magnum. Before testing was to be conducted, very thorough measurements of the rifle were taken so as to provide a benchmark for the testing which was to be undertaken. The first test was conducted using a bullet propelled by of Du Pont #4350 powder. This load provided of pressure. This load did not show any pressure or extraction issues with the new Mark V action but caused a slight sticking of the cartridge case in the Mauser style rifle design. Subsequent testing was performed using the same bullet and using a powder charge of Du Pont #4350 which increased by increments of for each test thereafter. The second testing which was conducted with the showed no signs of pressure nor issues with extraction even though the measured pressure was close to . Firing this load in the Mauser rifle led to a blown primer and extreme difficulty was experienced in extracting the spent case. Using of Du Pont #4350 the cartridge began to show signs of pressure in the Mark V action. However, the case did not stick and extraction was performed easily. Breech pressure was found to be between . Measurements of the spent case showed that the case had stretched at the belt a mere . The spent case from the test led to a slightly sticking case, which in turn led to a slight difficulty in opening of the bolt. Measurements from the case belt showed that the belt had expanded from to . The pressure generated by this load was . The fifth test was conducted used a load of of Du Pont 4350. Firing this load led to some difficulty in opening the bolt, and the case was extracted when opened. The belt of the case still measured . A difference in the diameter between the bolt head and the diameter of the barrel of per side was noted. No bulging of the bolt, receiver or the barrel was noted. Headspace was measured to be the same as prior to the testing. Further testing was conducted with a bullet lodged into the throat of the barrel. A cartridge loaded with the standard charge of of Du Pont 4350 and a bullet was fired into the back of the first bullet. It was found that both bullets exited the barrel. The primer had been pierced and the exiting gas entered into the bolt and hit the firing pin sleeve, which was loosened slightly. The bolt was opened by hand but the cartridge stayed stuck in the chamber. When the case was tapped out, it was found to be in good condition except for its pierced primer. It was found that the barrel, just in front of the receiver ring, had expanded from to . The diameter of the bolt head had expanded from to . The head space had increased from to . All other dimensions had stayed constant. This test was conducted 15 times. A test was conducted with a bullet lodged into the bore of the rifle and a grain bullet was fired into the back of this bullet. The result of this test found that the cartridge case head had expanded to . After these additional 15 tests it was found that the head space was set back only a mere .Action variants
The Weatherby Mark V action has remained virtually unchanged from the original design of Fred Jennie and Roy Weatherby which is a testament to the longevity of the design. Nevertheless, some refinement to the design has occurred over time. The Mark V action is a front locking, push feed, bolt action repeater. The lug arrangement allows for 54° bolt lift. This shorter bolt lift allows for increased clearance between the bolt handle and any accessories such as scopes that may be mounted on the rifle and arguably provides for faster cycling of the action. This compares favorably with the Mauser style rifles which have a 90° bolt lift. The bolt body, including the handle is made of a single piece of machined steel. To remove any play and reduce any chance of the bolt binding during the cycling of the action, the bolt head is smaller with the locking lugs being the same diameter as the body itself. The bolt has three gas vents along the bolt body which will allow for the venting of hot gases should the case not seal the chamber or if a pierced primer should occur. Shortly after production began in Germany, the bolt body was fluted to provide further positive feeding and extraction of the cartridges.9 Lug/Magnum
The 9 Lug/Magnum action was the original design which went into production and was also the design which was subject to the strength testing conducted by Weatherby. The original 9 Lug design lacked the bolt fluting and featured a safety mounted on the receiver. These rifles were made by PFI and J.P. Sauer & Sohn in the early 1960s. The refinements to the original design included a fluted bolt and a redesigned safety allowing for smoother feeding and extraction and a safer design. The 9 Lug action is considered the archetypal Mark V action. The 9 Lug design features a bolt head with nine locking lugs arranged in three rows with each row having three locking lugs. The Magnum Mark V bolt is the heaviest bolt available in a commercially produced rifle. The 9 Lug Mark V action is currently available in rifles chambered for Weatherby calibers. However, late production Sauer and very early Howa manufactured Mark V rifles in .30-06 Springfield and .270 Winchester were chambered in the 9 Lug Mark V actions. As the production run of these rifles was short such rifles are a rarity and considered collectibles.Varmintmaster
In 1963 Weatherby introduced a scaled down version of the 9 Lug design which was made specifically for chambering smaller varmint cartridges. The Varmintmaster was introduced together with the .224 Weatherby Magnum cartridge. The Varmintmaster was also chambered in the .22-250 Remington. This action featured six lugs arranged in two rows. The 9 Lug design was found to be too large and too long for use with varmint cartridges and this new action addressed this issue. The action features a shorter one piece bolt of a smaller diameter than the 6 Lug/Standard design. The Varmintmaster action was later used in the Weatherby Mark V Silhouette handgun as well.6 Lug/Standard
Before Weatherby launched its Rebated Precision Magnum (RPM) cartridge family in 2019 with the 6.5 Weatherby RPM, the 6 Lug design was only available in non-Weatherby cartridges and was offered beginning with the .30-06 Springfield and the .270 Winchester cartridges. Like the Varmintmaster (which predates the 6 Lug/Standard action design) it features six locking lugs arranged in two rows. The bolt is shorter and is considerably lighter than the 9 Lug/Magnum design. Coupled with a narrower receiver, the Standard Mark V action is better suited to most cartridges. This action type is also used by the Weatherby CFP (Center-fire Pistol) as it provided a greater flexibility as to which cartridges could be chambered. The Standard Mark V action was also used in the Super Varmintmaster rifle rather than the original Varmintmaster action due to cost and production constraints. The only Weatherby cartridges that are compatible with the Standard Mark V action are those of the RPM family (currently the 6.5 RPM and .338 RPM, the latter introduced in 2022).References
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