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The Technical Pedigree of the Short Fat Magnums

The Technical Pedigree of the Short Fat Magnums
The short fat magnum rifle cartridges have a “technical pedigree” in the form of a U.S. government patent.
When a company releases a new cartridge, it distributes press releases to tell us the “why it’s good” story. The Short Mags have another source, a “technical pedigree” in the form of a U.S. government document, a.k.a. a patent.

In the world of inventions, a patent is a government license granted to an inventor to protect his or her “intellectual property,” the legal term for an idea. For a set period of time, roughly 20 years in the United States, the inventor has exclusive rights to using the idea in return for disclosing the idea. However, few cartridge designs are patented.

The few that are patented are usually from the small, private inventor. Many small operations, having spent money on patent fees and attorney costs, never have a major manufacturer consider the idea, and we often never hear of them. However, one patented concept from a private inventor found spectacular success. The inventor holds several patents on related concepts due to the way the U.S. Patent and Trademark likes to divide one into several to avoid overly broad claims. I chose one that I thought best exemplified this concept of a cartridge’s “technical pedigree.” That patent, issued October 26, 1999, bears number 5,970,879 and the inventor’s name, John R. Jamison. I know him better as “Rick.”

Jamison’s patent gets right to the point. The title, “High-power Firearm Cartridge for Short-action Chamber and Bolt Assembly,” tells me immediately that Jamison was thinking of improved cartridge performance in compact rifles. Smack in the abstract (“condensed” version) of the patent we find two words that seem out of place in a technical document: “short fat.” However, these are important and are later described in technical terms by an aspect ratio, here being the mathematical relation between case length and case diameter.

Moving to the first section of the patent, we find “Background of the Invention.” Jamison’s opening lines complete the thoughts begun in the title and abstract:

“The present invention is directed to a…firearm cartridge of unique profile which makes it especially adaptable for use in a short-action firearm…. More particularly, the cartridge has unique length and diametric relationships which enable propellants to be burned more quickly and completely, thereby producing more energy and muzzle velocity for any given powder capacity than is possible with most previous cartridges having the same powder capacity.”

After a little background, the patent gets to the heart of the matter. Jamison states the cartridge has a ratio of overall case length to diameter (measured 1.25 inches from the base) of “no more than about 4.2.” It further states that to achieve desired propellant burning characteristics, that diameter is preferably “at least about 0.53 inch.” It also adds that the ratio of case body length (base to shoulder origin) to diameter should be “no more than 3.33.” Note the lack of exact numbers; this is classic patent strategy—claiming precise values opens the door to infringement attempts.

In the lengthy section detailing the “preferred embodiment”—patent attorney talk for the most likely form of the invention—Jamison lists a number of reasons why this case geometry is important. For me it was easy to pick out the “Big Three.” They are, with some space-saving edits:

“The upper limit on the [length/diameter] ratio maximizes cartridge diameter which places more of the propellant in proximity to the initial flame front produced by the primer.”

“The maximized diameter provides increased surface area at the front of the wider case portion…providing increased resistance to unburned powder granules…and reflecting more unburned granules rearwardly into the burning propellant where they are consumed.” His design elsewhere calls for a somewhat steep shoulder but one less than 40 degrees to facilitate this.

“The minimized cartridge length reduces the distance for the flame front to travel to ignite all the propellant.”

Jamison summarizes these three factors:

“The more efficient ignition and combustion resulting from these three effects turns almost all of the granules into a gas before they come out of the case, producing more energy more quickly while reducing the unburned granule mass which must wastefully be accelerated together with the bullet.”


A figure published in the Jamison patent shows how the short fat concept should shift the time-pressure curve for improved performance in compact rifles.

What sets this concept apart is careful attention to what happens in the first few microseconds of the firing cycle. To me, these present a very careful and astute study, but there is a figure included that attracted my ballistician’s eye. In his Figure 10 illustration (left), Jamison includes hypothetical time-pressure curves. Jamison’s diagram shows the curve for his concept, the “present invention,” with one he calls a “conventional cartridge.” Jamison writes:

“The pressure curves in FIG. 10 illustrate two advantageous results of the present invention. The more complete and quicker propellant ignition provided by the cartridge of the present invention produces a faster pressure rise time and more area under the pressure curve prior to the bullet exit. The area under the curve for a cartridge of the present invention is about 10% greater than the area for a conventional cartridge having the same propellant capacity. The increased area underneath the pressure curve illustrates the increased energy imparted to the bullet prior to muzzle exit.”

Although patents by their basic nature are complex documents, one can glean the key benefits of an invention with a little work. We don’t often see such a detailed technical overview of why a cartridge is built to a specific configuration. Here a person with a keen understanding of interior ballistics has laid out his findings for us.

Jamison’s “short fat” patents are the basis for the Winchester Short Magnums and the Remington Short Action Ultra Magnums. Next month I’ll look at the .300 WSM and how it lives up to this technical pedigree.

If you want to learn more about patents in the United States, enter “” in your browser’s address bar to find a great resource. 

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