Introduced in the Mark V rifle in 1968, the .240 Weatherby Magnum was the last of a dozen cartridges introduced while Roy Weatherby was alive. Physically, the .240 bears a strong resemblance to the British designed .240 Apex or .240 Belted Nitro Express as it is more commonly called. With a rim diameter the same as that of the .30-06, the Weatherby cartridge is suitable for any rifles with standard action lengths.
The .240 Weatherby Magnum represents about all the powder capacity that can be utilized with a 6mm bore with any degree of efficiency. When the 6mm Remington and the .240 Weatherby are loaded to maximum chamber pressures in barrels of equal length, the larger case will push all bullet weights 100 to 150 fps faster. Like other cartridges of low expansion ratio, the .240 needs a long barrel in which to reach its impressive performance.
The .240 Magnum is an excellent compromise in performance. Its relatively small bore size allows the use of lightweight bullets at extremely high velocity on varmints without excessive recoil, yet with a trajectory that will give any .224" centerfire a run for its money. Loading a 90 to 105 grain bullet in the Big Six transforms it into a fine deer and pronghorn cartridge.
With the exception of light, varmint weight bullets, powders of slow burning rate offer the best performance in this cartridge. Excellent choices are H4831, H450, and IMR-4831. With the lighter bullets H414, H4350, IMR-4350, IMR-4320, and H380 are better choices.
The 240 Weatherby was added to round out the Weatherby proprietary magnum line in 1968. It differs from other 6mm's in having a belted case with somewhat greater powder capacity. It is very similar to the 240 Belted Rimless Nitro Express introduced by Holland & Holland around 1923. Thus far, it is available only in the Weatherby Mark V bolt action rifle or through custom gunsmiths. It is an excellent cartridge and will push the 100 grain 6mm bullet with about 200 fps greater muzzle velocity than the 6mm Remington and around 400 fps faster than the 243 Winchester. However, a considerable portion of this ballistic advantage results from increased barrel length and loading pressure. It is important to allow plenty of barrel cooling time with this, and all, high intensity cartridges. It is a more effective deer and antelope cartridge than the other two. The principal detraction regarding the 240 Weatherby Magnum is that ammunition is expensive and difficult to find outside of the large cities. The 240 case has about the same capacity as the 30-06 and the rim diameter is also the same.
The 240 Weatherby is the most powerful of the 6mm cartridges. It represents the maximum performance that one can squeeze through a 6mm tube with modern powders. The 244 H&H Belted Rimless Magnum, based on necking down the 375 H&H Magnum case, will hold more powder but doesn't produce any improvement in ballistics. The late Roy Weatherby built a successful proprietary gun business on the basis of good product plus the all-important element of ballistic one-upmanship. The weatherby magnum cartridges have traditionally offered higher velocity and energy than their standard factory counterpart. The 240 was born of this same tradition. Of course Remington, Winchester, Norma, et al have their own magnum line in various calibers, and Winchester offered the 300 and 375 H&H Magnums before WWII. However, Roy Weatherby was the first to really popularize this British innovation in the U.S. He convinced the American shooters that it was something they truely needed. For the hunter who must have that extra edge in a 6mm rifle, the 240 Wetherby Magnum is the cartridge of choice.
.240 Weatherby Magnum