October 18, 2019
Prior to 1956, the average American hunter considering a dangerous-game hunt was hard-pressed to find a “born in the USA” cartridge in a production rifle that checked all the boxes. The .375 H&H was popular, but British; the proprietary .378 Weatherby certainly had the energy (over 5,500 ft-lbs at the muzzle), but like the .375 H&H, it was shut out of some hunting areas because some governments banned the use of any rifle under .40 caliber for hunting dangerous game. Missing was a cartridge that reliably functioned in standard American bolt rifles available in the mid-1950s, that produced over 4,500 ft-lbs of muzzle energy with a 500-grain bullet, and that satisfied international game regulations.
Winchester filled the gap in 1956 with the 458 Winchester Magnum. Starting with the H&H basic pattern, the case was shortened to 2.5 inches to work through the Model 70 bolt-action rifle when the bullet was loaded to under 3.34 inches—that’s .30-06 length. The .458 Win. Mag. looks almost too puny to achieve the energy levels needed for an African cartridge, yet it did. How? Pressure, that’s how.
The popular towering British cartridges used for hunting dangerous game were developed for Cordite, a primitive smokeless propellant formed into long thin rods requiring a long case. Cordite always had one dirty little problem: sensitivity to temperature extremes.
Cartridges using Cordite could spike pressures to very undesirable levels in temperatures of 100 degrees Fahrenheit. Thus, cartridges were always factory-loaded at very modest pressures to keep them safe in summer heat. Derived from British proof standards, the .470 Nitro Express (NE) has a maximum average pressure (MAP) assignment on SAAMI equipment of only 41,000 psi. That’s less than the .30-30 Winchester.
By the time the 458 Winchester Magnum appeared, there were plenty of modern, compact propellants that behaved in extreme temperatures. The .458 Win. Mag. was designed for modern bolt actions, so it has an assigned MAP of 60,000 psi, the same as the .30-06. This enables the compact .458 Win. Mag. to achieve near parity with the “big Brits” in this class like the .450 NE, the .500/450 NE, and the .470 NE.
There are plenty of excellent factory loads available, and the .458 Win. Mag. is also widely handloaded. For this compact cartridge, the handloader will look to propellants in the middle of the burning rate range as they would for any other straight-walled rifle case. The slow-burners are simply too bulky, reducing velocity potential.
Which mid-rate powder? When we last did .458 Win. Mag. transducer data for 500-grain bullets at Speer, most of the “usual suspects” posted velocities within 40 fps of each other with better-than-average variations in pressure and velocity. Simply use what you have.
Today’s excellent selection of improved dangerous-game bullets has both helped and hurt the .458 Win. Mag. The first homogeneous solids (no lead) from three decades ago were much longer than lead-core FMJ bullets of the same weight and ate into an already limited propellant space. This could drop .458 Win. Mag. muzzle velocities to 1,900 fps or less, compared to roughly 2,090 fps with lead-core bullets. Fortunately, the homogeneous solids have evolved nicely over the last 30 years. They are still longer, but smart engineering allows them to achieve velocities on par with conventional bullets.
Handloaders should plan on using magnum primers with the .458 Win. Mag. Even though the case capacity is not huge, reduced loads have shown a tendency to “click-bang”—a tiny but detectable delay between the firing pin strike and the satisfying “boom” of complete ignition. Think of a flintlock firing. Developing softer-shooting practice loads is not a slam-dunk with this cartridge.
Once I believed this was all about how fast pressure drops when a .45-caliber bullet moves just a little. The .375 H&H has a similar capacity but a smaller bullet. I’ve shot hundreds of reduced-velocity handloads in .375 H&H and never had a click-bang, yet you can create them almost on demand in the 458 Win Mag.
I see something different with the 458 Win Mag chamber that could explain this. The industry chamber drawing for the .458 Win. Mag. shows its throat having zero freebore but a very long leade, the tapered section of throat over which the rifling goes from zero depth to full depth. With few exceptions, total throat lengths in sporting rifle chambers are under 0.300 inch. The .458 Win. Mag. throat is over 1.1 inches, tapering from 0.469 inch to 0.458 inch. Along with the obscure 9.3x62mm cartridge, that is the longest chamber throat in the SAAMI manual by nearly 0.5 inch.
The people I knew suffering the highest occurrence of click-bangs were loading cast bullets that were designed for the .45-70 Government. The .45-70 has a short, abrupt (12 degrees) leade, so its bullets require a step down between the bearing surface and the origin of the nose. Original .458 Win. Mag. factory bullets were not stepped but rather tapered to fit the odd throat, and on firing, encountered rifling resistance early. Stepped bullets have to go farther down the throat before encountering much resistance. That could create a click-bang.
Click-bangs don’t occur with full-power handloads using bullets close to factory weight, so don’t let this put you off. Handloaders can create reduced .458 Win. Mag. loads, but they will take more work. If you are using step-nose cast bullets (most are), try seating long so bullets engage the rifling sooner. You need a firm crimp, so crimp into an exposed lube groove. Also stick with rifle propellants; shotgun and handgun propellants won’t have the bulk you need.
The .458 Win. Mag. is still a viable cartridge, even with the ready availability of more dangerous-game rifles and cartridges than ever before. It’s used wherever in the world dangerous game is hunted because when you compare performance to price, it works.