The .300 Winchester Short Magnum

December 18, 2018 By Allan Jones

Last month I talked about something that almost never happens: a patented cartridge design that excelled as a commercial product. Several patents by John R. Jamison, or “Rick” to many of us, describe the concept. He sets forth a compact rifle cartridge case design that he describes as “short/fat” and intended for short-throw bolt-action rifles. I referred to the patent as a “technical pedigree.”

The patent details ranges of dimensions and ratios between them that cooperate to generate excellent velocities by shortening the pressure’s time to peak and by burning more of the propellant inside the case rather than in the bore. Quite a few cartridges now fall under this design concept, but the one that saw a meteoric rise in consumer acceptance was the .300 Winchester Short Magnum (WSM).

The WSM series uses a non-belted case. I’ve spun enough ink regarding my reloading concerns with belted cases that don’t need a belt, so I’ll not repeat myself here. Suffice it to say, the .300 WSM was already one-up on my scorecard before we squeezed the first trigger.

A lot of verbiage has described the Short Magnums as “based on” the .404 Jeffery case, but the better description is that they were “inspired by” the old British workhorse. The .404 Jeffery, at 0.545 inch, is smaller ahead of the rim than either Winchester or Remington short/fat designs; they are 0.555 inch and 0.550 inch respectively.

My Speer development team was keen to jump into learning about a new cartridge from the ground up. However, we were under pressure to get data out and had to temper our enthusiasm—some things we learned came later. To get data online quickly, we focused on the most popular hunting bullet weights. We tested three Speer Hot-Cor Spitzers (150, 165, and 180 grains) plus the 165-grain Trophy Bonded Bear Claw, available as a component then. We also screened the 200-grain Hot-Cor—more on this later.

A Very Efficient Concept

In analyzing thousands of pressure data sheets, I was accustomed to seeing traditional magnum rifle cartridges show ballistic consistency issues at lower charge weights. The extreme variation (or EV—the highest value in one data set subtracted from the lowest) for the lower charge weight pressures was usually conspicuously higher than those of charge weights closer to maximum pressure. Most “tall/skinny” .30-caliber cases from the .30-06 up show this. It was so pervasive that we called it “normal” and even adjusted start charge recommendations to avoid these pressure regimes if needed.

Immediately, we saw this low-pressure variation was consistently minimal in the .300 WSM. It was seldom greater than EVs of any higher charge weight and definitely averaged less than other .30-caliber magnums. Incidentally, we saw this same behavior in the .300 Remington Short Action Ultra Magnum that also falls within Jamison’s patent parameters.

If “efficiency” is getting the most output from the least input, Jamison’s concept seems to be working. Before I retired I analyzed the .30 calibers from the .30-06 though the .30-378 Weatherby. I selected lab data using the Speer 180-grain Hot-Cor Spitzer SP and looked for loads producing the highest velocity in a 24-inch SAAMI test barrel and how much propellant it required to reach said velocity. We used the propellants available to us prior to the release of the Speer Reloading Manual #14 in 2007. I referenced everything back to .30-06 velocities and charge weights, looking for what percent increase in propellant charge gave what percent increase in velocity over .30-06 velocities. The results were interesting, and in some cases they were eye-opening.

Setting the .30-06 as a baseline, the well-established .300 Winchester Magnum uses 37 percent more propellant than the .30-06 to attain a 13 percent increase in velocity. That’s no bargain. On the other hand, the .300 WSM uses only 14 percent more fuel than the .30-06 to get an 11 percent velocity gain, a reasonable return. The WSM also posted a maximum average velocity only 49 fps less than its bigger, older brother using 14 grains less propellant.

We later added lighter bullets. Normally, developing loads for the .30-caliber magnums with varmint-weight bullets was a painful slog through inconsistencies, but the .300 WSM was very well behaved with 110- and 130-grain bullets. We also developed reduced-recoil loads using Accurate 5744 that were very consistent. One of the 150-grain loads roughly duplicated .30-30 velocities with superb consistency in pressure and velocity. You can train new shooters with that load, and as they grow in stature and experience, increase the recoil incrementally.

If I could do it over, I would spend more time with 200-grain loads. With management wanting data yesterday, we skipped them because our early screening showed unimpressive improvement over 200-grain .30-06 velocities. Today, I would change both my mind and the copy I wrote about heavy bullets in the Short Mags. Hodgdon’s online data shows they broke 2,800 fps with a 200-grain bullet with several propellants that we were not able to get before the Speer Reloading Manual #14 had to go to press.

Does What It Promised

Increases in velocity over that of the .30-06 are virtually linear with the increase in propellant, unlike more conventional “tall/skinny” .30-caliber magnums.
Variations in start load pressures are much less than in most “tall/skinny” magnums.
Shows excellent linear tracking of pressure against charge weight in bullet weights from 110 to 200 grains.
A lack of “pickiness” about propellant burning rates. 4350-class propellants often give velocities similar to 4831-class propellants.

At one point, the .300 WSM was “another .300 Magnum” to me; today, I feel very differently, especially when addressing handloaders looking for their first .30-caliber rifle. With confidence the handloader can create loads whose performance keeps pace with cartridges from the .30-30 through the .300 Win. Mag.