.424 Lee Magnum (P.O. Ackley Data)

.424 Lee Magnum (P.O. Ackley Data)

The .424 Lee Magnum is a powerful, yet rare, wildcat cartridge created and popularized through the experimentation of famed gunsmith and ballistician P.O. Ackley. Like many of Ackley’s wildcats, this cartridge was designed with performance in mind, blending innovative design with the capabilities of modern powders and bullets available at the time. The .424 Lee Magnum sits in a unique niche: a big-bore cartridge optimized for hunting large game with impressive terminal ballistics and relatively flat trajectories compared to traditional heavy-hitters.

The cartridge is centered around a true .424-inch bullet diameter — a relatively uncommon bore size — and utilizes a blown-out and necked-up case with a sharp shoulder angle, which is a common characteristic of many Ackley creations. Its case capacity is substantial, allowing it to push heavy 350- to 400-grain bullets at high velocities, often approaching or exceeding 2,000 feet per second, depending on barrel length and powder charge. This combination results in energy levels exceeding 3,000 foot-pounds at the muzzle, making the .424 Lee Magnum suitable for elk, moose, and large bear species.

Like many wildcat designs, commercial support for the .424 Lee Magnum has always been limited. Dies are typically custom ordered, and rifles chambered for it are often built from strong bolt-action platforms such as modified Mausers or Springfield 1903s. The rebated or straight-wall nature of the case (depending on variation) requires careful attention when forming brass. Reloaders often use modified .375 H&H or other belted magnum brass as a starting point, then fire-form to final dimensions. Case trimming and neck turning may also be necessary, depending on chamber tolerances.

Reloading data is primarily sourced from Ackley’s books and articles, with favored powders including IMR 4350, H4831, and Reloder 22. Because of the large bore diameter and relatively short bullets compared to their mass, seating depth and neck tension are critical to ensuring both safety and performance. As always with wildcats, users are encouraged to start low and work up carefully, observing for pressure signs.

The .424 Lee Magnum exemplifies Ackley’s vision of ballistic experimentation — pushing the limits of what custom cartridges could do before the rise of commercial mega-magnums. While few modern hunters use it today, it holds historic significance and appeal for those interested in forgotten wildcats or building unique custom rifles for large game applications.

For those seeking to revive or experiment with the .424 Lee Magnum, P.O. Ackley’s original data and commentary remain invaluable, offering foundational guidance on charge weights, bullet pairings, and case modification strategies.