6.5/300 Weatherby Magnum

The Big Kahuna of Our Current “Most-Favored” Bore Size

By John Barsness

Any rifle loony who’s spent much time awake in the 21st Century knows the 6.5mm dominates new cartridge development these days. Whether they’re in love with this trend is another question altogether. Opinions range from strongly suggesting the vintage 6.5×55 Swede and .264 Winchester Magnum do the same things as the latest crop, to “liking” any new 6.5 round on Facebook. Once again the generations are at war, and perhaps no cartridge symbolizes the conflict more than the 6.5/300 Weatherby Magnum.

First, many 21st Century shooters consider the 6.5/300’s case design to be terribly “last century.” Today’s trend is toward shorter, fatter and belt-free, but the 6.5-300 (as its name implies) is the .300 Weatherby Magnum necked down. But hey, it’s a Weatherby cartridge, and nothing says “Weatherby” like a long belted case with the distinctive rounded shoulder.

The second phase of the generational conflict involves exterior ballistics — what happens as bullets fly from the muzzle to the target. Even though the 6.5×55 appeared in 1892, most American shooters suddenly “discovered” the 6.5 during the 1990s. The effects of gravity on bullet flight are constant and predictable, so laser ranging decreased the need for a “flat” trajectory, the major reason for high muzzle velocity — the dominant desire of rifle shooters ever since practical smokeless rifle powders appeared in the late 1800s.

Instead, the Great God Ballistic Coefficient started taking over, because high BC bullets reduce wind drift. But unlike gravity, wind isn’t predictable either in speed or direction. While increasing a bullet’s muzzle velocity 10 percent — say from 3,000 to 3,300 fps — reduces wind-drift about 14 percent at 500 yards, increasing the same bullet’s BC coefficient 12 percent will decrease wind-drift just as much without increasing muzzle velocity. This eliminates the “recoil penalty” involved in extra velocity.

Increasing BC by 33 percent (not uncommon when comparing 140-gr. flat-based, softpoint 6.5mm bullets to high-BC 140s) reduces 500-yard wind-drift even more. This is exactly why smaller 6.5mm cartridges have become increasingly popular: Their high-BC bullets allow us to decrease wind-drift at longer ranges without getting the snot kicked out of us.

Proven on paper and game: These bullets and powders (below) are John’s preferred components for his three loads.

Best of Both Worlds?

However, many American hunters still firmly believe in high muzzle velocity for longer-range shooting. And if we combine high muzzle velocity with high BC bullets, won’t this reduce wind-drift even more? Yes, it does — and fits right in with another American ideal — having it all.
This is exactly what the 6.5/300 Weatherby accomplishes over smaller 6.5s: A high-BC bullet in the 140-gr. class at 3,300 fps drifts about 30 percent less at 500 yards than a flat-based spitzer at 3,000 fps. Yeah, the 6.5/300 kicks more, but not obnoxiously so. My Weatherby Ultra Lightweight weighs 8 lbs. scoped and its recoil feels similar to a 7mm Remington Magnum with 160-gr. bullets.

Recoil can be also reduced by using lighter bullets. However, there really isn’t much ballistic sense in using bullets much lighter than 130 grains in any 6.5 rifle intended for longer-range shooting. Reducing bullet weight also reduces BC, increasing wind-drift.

All three shoot well under an inch from his Weatherby Mark V Ultra Lightweight.
Who says you can’t have it all?

Touchy Pressures

Plus, there’s not much published, pressure-tested reloading data for the 6.5/300, and none of it includes bullets lighter than 127 grains. This isn’t due to a lack of demand (the cartridge appeared in 2015 and is pretty popular among hunters), but rather the simple ballistic fact that extremely high-velocity cartridges can be a little persnickety about pressure. During considerable load development I haven’t encountered any problems, but any cartridge with lots of powder capacity for the bore diameter will tend to be more “touchy” (as Hodgdon’s Ron Reiber describes the 6.5/300) than modest, well-mannered cartridges like the .308 Winchester.

The development of more really slow-burning powders has helped faster magnums considerably in recent years, partly because so many newer propellants are more temperature-resistant. It’s hard enough to handload the rounds the late P.O. Ackley called “overbore” without worrying about whether a warm day will blow primers.

The loads I’ve settled on in my rifle are the 127-gr. Barnes LRX and 91.5 grains of Hodgdon US869 for around 3,450 fps, the 140 Nosler Partition with 90.0 grains of US869 (3,300 fps), and the 143 Hornady ELD-X with 78.0 grains of IMR8133 (also 3,300). All were developed in Weatherby cases with CCI 250 magnum primers.

The 140 Partition may seem a little strange amid this mix, but has a decent BC for a flat-based softpoint. It also holds together on big game at close range, yet expands easily at longer ranges due to the relatively soft lead alloy used in the front cores of all Partitions.

In my experience, the 140 Partition also shoots accurately in just about any 6.5mm rifle, and is more forgiving of seating depth than most high-BC boattails. The 127 LRX and 143 ELD-X both required some experimentation with deeper seating to match the Partition’s 100-yard groups.
Despite the generational contradictions of the 6.5/300 Weatherby Magnum, it does come pretty close to having it all.

John Barsness’s Big Book of Gun Gack: The Hunter’s Guide to Handloading Smokeless Rifle Cartridges can be ordered through www.riflesandrecipes.com, P.O. Box 579, Townsend, MT 59644-0579, (406) 521-0273.

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