Good Question. In gunsmithing terms a wildcat is any cartridge that is not now, nor has ever been a factory offered caliber.
For instance; the 243 Rockchucker was the most popular of the 243/6mm wildcats available. Then one fine day Winchester tm introduced the 243 Winchester tm as a factory answer to the popularity of the 243/6mm wildcats that were thriving at the time, this spelled death for the 243 Rockchucker, although there are still people shooting that wildcat.
Some Cartridges break this rule (factory production) through no fault of their own. The 35 Whelen was a wildcat for about 75 years before Remington decided to standardize it a few years back. Unfortunately this often ruins the reputation of a cartridge. In the case of the 35 Whelen the ammunition that is factory loaded for it today is low pressure and therefore low velocity, I have heard it said that you could use a sundial instead of a chronograph on such loads.
This low pressure loading is done for two reasons, first to accommodate the fact that the most recent factory gun manufactured in 35 Whelen was a pump action, a relatively weak design when compared to modern bolt guns. The second reason is that many of the guns built in this caliber over the past 75 or so years are of questionable strength. By offering low pressure ammo the manufacturer minimizes liability. A well built modern commercial bolt action rifle will handle substantially higher pressure and therefore give much better ballistics.
What's an Improved Cartridge?
The most famous of all are the cartridges designed by P.O. Ackley. He took factory offerings, expanded the body to minimum body taper, and changed the shoulder angles. The idea is to give you more velocity from the case yet allow you to fire factory ammo in the chamber. This allows you to fire form the brass easily, and if traveling you could buy factory ammo to shoot in the rifle in a pinch.
It is purely my opinion that improved cartridges are not true wildcats. That is the final definition of a wildcat, that it must be formed in dies, and no factory cartridge can safely be fired in the wildcat chamber.
Who can design a wildcat?
Anybody who has the desire and the wherewithal (is that a word?).
It's a way of having something that no one else in the whole world has. It's a challenge to work out the details. Its interesting to try to design a cartridge that will do something that no other cartridge will do in that same way. Wildcatting is just plain fun.
What do you need to design a wildcat?
Nothing, well almost nothing. You need a goal, what niche do you want your cartridge to fill?
Then you need some basic knowledge, what type of firearm will work with your wildcat?
Seek advice from a competent gunsmith. A chamber reamer will have to be made to meet your needs. Then a test gun is built, its OK to make it a nice gun because even if the wildcat turns out to be less than you expected you can rebarrel it to something else, no one will ever know but you. Then you will have to work up loads this can take a lot of time, with a true wildcat there is no reloading manual to follow (some manuals contain data for the more popular wildcats) . It is also very rewarding to test a new wildcat and get more than you bargained for, and I have.
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