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The Ultimate Reloading Manual
Wolfe Publishing Group
  • reloading manual
  • alliant reloading data
  • reloading brass
  • shotshell reloading
  • bullet reloading
The Ultimate Reloading Manual
hodgdon load data

.300 Winchester Magnum

Author: Brain Pearce / Wolf Publishing Co.
Date: Nov 15 2018

When the .300 Winchester Magnum was introduced in 1963, it was technically a short magnum, although that term has completely different meaning today. For example, competing magnum cartridges such as the .300 Holland & Holland Magnum and the .300 Weatherby Magnum both required the long .375 H&H length (3.600 inches) action, whereas the new .300 Winchester was shortened and designed specifically to function in 30-06 (3.340 inches) length actions. Its popularity grew instantly with every major rifle company offering it as standard chambering, and ammunition companies quickly offered loads. In spite of many .30- caliber magnum cartridges appearing in the last couple of decades, the .300 Winchester Magnum still enjoys widespread use among big game hunters, and long-range shooters, and it serves the U.S. military. Current ballistics generally advertise cup-and-core 150-grain bullets at 3,290 fps and 180-grain bullets at 2,960 fps. However, there are many specialized loads that include additional bullet weights and designs that increase the cartridge’s versatility.

The .300 Winchester Magnum is based on the H&H belted magnum case and features a 25-degree shoulder. Most rifles will permit the case to be sized to change the headspace control from the belt to the shoulder, which usually increases accuracy and extends case life. However, if this practice is used with hunting ammunition, be certain all cartridges will chamber easily in your rifle. A more popular option is to full-length size cases that will be used in the field. Furthermore, if hand loads are to be used in multiple rifles, full-length case sizing is highly recommended. When reloading cases for auto-loading or lever-action rifles, a small base sizing die is strongly suggested to assure reliable and easy chambering of hand loaded ammunition.

The industry maximum average pressure was traditionally established at 54,000 CUP; however, it has been updated to 64,000 psi using piezoelectric pressure measuring methodology. Hand loaders should take note that case capacity varies significantly from one manufacturer to another. As a result loads that are within industry pressure guidelines with a given case can result in excess pressure when the same load is assembled in another case. In some instances the maximum powder charges will not even fit in cases with less capacity. This is a good reminder to never begin with maximum powder charges. Additionally, data presented with spherical powders should not be reduced below the suggested start charge weights.

As indicated, the maximum overall cartridge length for the .300 Winchester Magnum is 3.340 inches. All loads here were kept within this length and were tested in a Ruger M77 MK II rifle with a 24-inch barrel. However, the Sierra 240-grain MatchKing HPBT bullet seated to 3.550 inches and was fired in a single-shot mode in a Winchester Model 70 with a 26-inch barrel that was throated to accept cartridges of this length.

While standard large rifle primers can give reliable ignition with select powders and can lower extreme spreads, for reliable ignition wen used in all temperatures (especially sub-zero) and with all powders, it is highly recommended to use large rifle magnum primers such as the Federal 215, which was used to develop the accompanying data.