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The Ultimate Reloading Manual
Wolfe Publishing Group
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The Ultimate Reloading Manual
hornady superperformance

Ackley Improved Cartridges

Author: John Barsness / Wolfe Publishing Co.
Date: Jun 21 2005

“Improved” cartridges have been around ever since Lisle Kilbourn cut a .22 Hornet chamber with a sharp shoulder and fired a standard Hornet case, resulting in slightly more powder capacity – and velocity. The process has since been applied to hundreds of cartridges. In fact, some factory cartridges are improved versions of other factory cartridges, notably the .300 Weatherby Magnum, an improved version of the .300 H&H Magnum.

These days it seems that every time we turn around, another cartridge has been “Ackley Improved.” Gunsmith P.O. Ackley experimented with improving various rounds in the 1950s and 60s, all with a 40-degree shoulder. He found that most of the time the process wasn’t worth the trouble, but his name caught on and today is applied to any improved cartridge with a 40-degree shoulder, whether P.O. ever made one or not.

There are advantages and disadvantages to Ackley Improving any cartridge. The most obvious advantage is more powder room, hence higher potential velocity. Just how much extra velocity is possible depends on the original cartridge. Rounds with tapered bodies and sloping shoulders gain the most, the reason the earliest improved wildcats were based on rounds such as .22 Hornet, .250 Savage and .300 H&H Magnum.

Here we must remember that potential velocity only increases at one-fourth the rate of any powder capacity increase. The .300 Weatherby Magnum gains about 13 percent in powder capacity over the .300 H&H, which translates into a 3.2 percent gain in muzzle velocity, everything else being equal. This means that if a 180-grain bullet can safely be pushed to 3,000 fps in a .300 H&H, it can be given about 3,100 in the .300 Weatherby – in an equal-length barrel at the same pressure.

I emphasize the last because .300 Weatherby factory ammunition has always been advertised as gaining 250 to 400 fps over the .300 H&H. This is because .300 H&H ammunition has been loaded to much lower pressures for many years, especially when compared to original .300 Weatherby ammunition, which was stoked to the gills.

But look up both cartridges in a modern loading manual that pressure- tests each round, and you’ll find the difference isn’t all that much. Nosler’s manual, for instance, lists the top velocity for a .300 H&H 180-grain load as 3,023 fps, while with the .300 Weatherby the top 180 load gets 3,185 fps.

This seems to favor the .300 Weatherby by more than 100 fps, but now let’s look at barrel length. The H&H was tested in a 24-inch barrel, the Weatherby in a 26-inch tube. Most magnum cartridges gain or lose about 30 fps per inch of barrel. Subtract 60 fps from the Weatherby data, and we get 3,125 fps, 102 fps faster than the .300 H&H data.

Another advantage of improved cartridges is that, unlike other wildcats, factory ammunition can still be fired if necessary. Also, the 40- degree shoulder of the Ackley Improved rounds cuts down on case stretching enormously, so cases rarely have to be trimmed. This can be a considerable advantage to a varmint shooter, for instance, who often loads hundreds if not thousands of rounds a year.

The disadvantages of the Ackley Improved cartridges start with fireforming. This is easily done in a varmint rifle, where hundreds of rounds of factory or conventional handloads can be shot at prairie dogs, but not so easily (or economically done) otherwise. Also, most Ackley chambers are cut by running a reamer into a conventional chamber. This results in a new chamber that’s slightly loose with factory brass. If a factory round or newbrass handload is fired here, the case is driven into the chamber by the firing pin, then stretches upon firing, often ruining the case. The solutions are to seat a bullet out so that it contacts the lands, thereby holding it against the blow of the firing pin, or to form the case with some fast-burning powder under a loose “bullet” of uncooked cornmeal. Tilt the rifle skyward while loading the chamber, and pop! New case!


An even better solution is to have the barrel turned back one thread before rechambering, so that the chamber exhibits a slight “crush fit” with new brass. Then any sort of factory or new-brass handload can be fired, and cases will be formed perfectly.

Another disadvantage to improved rounds is that any rechambered rifle loses at least $100 of its value right then and there, since wildcat rifles are not popular among the general public. A custom rifle by some known maker, however, will hold its value better.

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