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Custom Cartridges: Design Considerations

Custom Cartridges: Design Considerations

Good cartridge design always provides positive headspace control, like the very modern-looking shoulder of the venerable .416 Rigby.

I suspect that Shooting Times readers know that a "wildcat cartridge" is one created from an existing cartridge by changing dimensions other than the basic head. It is often accomplished in a hobby setting and, of course, does not have pressure standards as do commercial cartridges. Some popular wildcats evolved into standard industry cartridges, such as the .22-250 Remington, .25-06 Remington, and .35 Whelen. Many wildcats were conceived to provide higher velocity, but that's not the only reason to design a cartridge for your special needs.

In coming up with these variants, someone hopefully had a plan. To paraphrase a line from the movie Jurassic Park, "Just because you can do it doesn't mean you should do it."

Pressure Considerations & Case Strength
If a major company in the firearms industry develops a cartridge, its designers will engineer the new case to meet the planned pressure levels. It is the hobbyist who sometimes fails to include this important factor.

I looked at a wildcat based on the .30-30 Winchester case that, from the data provided, had to be operating in the 52,000 CUP range or higher. The reloader, who asked my help, was pleased with the performance but was concerned over short case life due to case stretch just forward of the head. The answer was pretty clear. The .30-30 Win. has a maximum average pressure (MAP) assignment of 38,000 CUP, so the cases do not require a lot of extra metal to fulfill their intended purpose and were too thin for a 38-percent increase in pressure. I suggested he sacrifice a little case length and consider the .375 Winchester case. That cartridge has a MAP of 52,000 CUP and thicker walls.

If you wildcat, choose a parent case that's up to the task, one designed for the pressures you anticipate.

Reasons For New Designs
The most common reason to design a new cartridge--commercially or privately--is improved performance. The popular Ackley Improved cartridges were generally very well-conceived and achieved a performance boost by reducing the case taper and increasing a standard cartridge's shoulder angle to as much as 40 degrees. Ackley's writings suggest that these designs were intended to give a performance gain with the heaviest bullet and the slowest propellants. If you shoot 0.284-inch, 120-grain bullets, the original .280 Remington will not yield much to the fine .280 Ackley Improved. If you shoot the heavier .28-caliber bullets, the Improved version begins to pull ahead.


Not all capacity increases accomplish as much as their creators hoped. By necking down a larger case to hold smaller bullets, there is a point of diminishing returns defined by the propellants available. When the A.O. Niedner Rifle Company necked down the .30-06 to create the .25 Niedner (basically today's .25-06 Remington) around 1920, the cartridge was handicapped by the propellants of the day. They were too fast-burning for the large case and small bore. The .25-06 did not come of age until slower reloading propellants like Hodgdon 4831 became available after World War II.

Some wildcats solve other problems. Based on the .30-06 case, the .35 Whelen shared its parent's shoulder angle of 17.5 degrees. The larger neck diameter reduced the surface area of this shallow shoulder, and in its wildcat days, some case brands failed to give enough support against the blow of the firing pin. If the cartridge managed to fire, accuracy was often lackluster and case life suffered. The variants of the .35 Whelen Improved offered steeper shoulders between 30 and 40 degrees that gave better headspace support, longer case life, and much better accuracy. There was not much performance difference unless you count cutting group sizes by up to half as "performance." (I do.)

I believe Frank Barnes coined this term back in the 1960s. He investigated the technique of shortening a larger case to actually reduce velocities for a particular purpose or to fit a certain gun platform. One popular Barnes wildcat was the 458x2-inch, sometimes called the .458 American. It was the .458 Winchester Magnum case shortened 1/2 inch and loaded to imitate an up-loaded .45-70 yet work in a standard bolt-action rifle. The shorter case allowed consistent ballistic performance with energy levels appropriate to most North American game, and it would work in a short-action bolt rifle.

Among the category of "mildcats" is a unique group of cartridges designed for cast bullet benchrest competition. One of my friends was into this sport and had a .30-caliber rifle built for it. The cartridge was the .308 Winchester case with the shoulder pushed back, extending the neck for precise bullet alignment. This alteration also created a smaller propellant space better suited to the light propellant charges and modest pressures common to cast bullet shooting.

Planning The Design
After safety, understanding the action type to be serviced is the top consideration. Usually the issue of action compatibility is addressed in the selection of a parent cartridge that fits the boltface of the action that receives the wildcat. Beyond that, I would offer the following as things to consider, assuming that you've chosen a parent case that can handle the pressure. In no particular order, they are:

1. Avoid the belted case if possible. Although handloaders can deal with the belt, it's just as easy to start with one of the newer nonbelted cases, such as the Remington Ultra Mags. The .375 Ruger has a nonbelted, nonrebated head that fits boltfaces cut for the classic Holland & Holland Magnum case head. This one grabbed my attention for that reason.

2. Don't cheap out on the case neck length. A wise guideline is to make the neck close to the bullet diameter; if you are building a .30-caliber wildcat, try to keep the case neck around 0.300 inch long. A longer neck often means you can avoid a custom coaxial bulletseater.

3. Watch the body taper. Some actions may not have the proper guide rail dimensions for reliable feeding. Correcting this is possible but is tedious work and, should you remove too much metal€¦, well, you get the picture. Some body taper is always necessary in bottlenecked cartridges.

4. Match case capacity to the bullet weights you plan to use. If you want to shoot 150-grain, .30-caliber bullets, a huge case will probably not produce the ballistic efficiency you need. In developing reloading data for the .30-caliber "megamagnums," we found that bullets under 180 grains were not as consistent in velocity. Although the loads were safe, there was simply more shot-to-shot variation than I like.

5. Should you create a big-game cartridge that can produce excep

tional velocities, make sure you can get component bullets that are up to the stress of extreme velocity. A conventional 150-grain bullet that expands perfectly at .30-06 velocities can act like a varmint bullet at 3,500 fps.

Creating cartridges is a great adventure for the advanced handloader. Just keep it safe and sane, and don't expect to get rich from your invention!

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