A wildcat cartridge, often shortened to wildcat, is a custom cartridge for which ammunition and/or firearms are not mass-produced. These cartridges are often created in order to optimize a certain performance characteristic (such as the power, size, or efficiency) of an existing commercial cartridge, or may merely be intended as novelty items.

Developing and using wildcat cartridges does not generally serve a purpose in military or law enforcement; it is more a hobby for serious shooting, hunting, gunsmithing, and handloading enthusiasts, particularly in the United States.^[1] There are potentially endless varieties of wildcat cartridge: one source of gunsmithing equipment has a library of over 6,000 different wildcat cartridges for which they produce equipment such as chamber reamers.^[2]

Often, wildcats are commercially sold rounds that have been modified in some way to alter the cartridge's performance. Barrels for the caliber are originally manufactured by gunsmiths specializing in barrel making. Generally, the same makers also offer reloading dies, tools to custom-load bullets into cases. Because changing the barrel of a gun to accommodate custom cartridges requires precision equipment, most wildcats are developed by or in association with custom barrel makers. Ammunition is handloaded, using modified parent cases and the gunsmith-provided wildcat dies. Generally, the supplier of the barrel or dies will also provide the buyer with basic reloading data, giving a variety of powders, charge weights, and bullet weights that can be used for developing loads. Handloaders use the data to develop a load by starting with minimum loads and carefully working up.^{[citation needed]}

Wildcat cases and cartridges can be found for sale, but only from small makers. Larger manufacturers usually do not produce wildcats because there is such a limited market for them and because there are no established CIP (Commission Internationale Permanente Pour L'Epreuve Des Armes A Feu Portatives - Permanent International Commission for the Proof of Small Arms) or SAAMI standards, which causes liability concerns.^{[citation needed]}

This section needs additional citations for verification. (January 2021)

Wildcat cartridges are developed for many reasons. Generally, the goal is to optimize some characteristics of a commercial cartridge in a given context. Higher velocities, greater energy, better efficiency, greater consistency (which yields greater precision), and complying with a minimal permitted caliber or bullet weight for the legal hunting of certain species of game in a particular jurisdiction are the top reasons. The sport of metallic silhouette shooting, has given rise to a great number of wildcats, as several rifle rounds are adapted to fire from a handgun.^[3] In using autopistols for hunting or competitive shooting, improved feeding of softnose or hollowpoint bullets is also an issue; the bottlenecked .45/38, for instance, was created because the straight-cased .45 ACP had trouble feeding hollow points.^[4]

Wildcat cartridges are generally developed because:

• Higher velocities can be obtained by increasing the case capacity or reducing the caliber.
• Greater energy can be attained by increasing the caliber or the case capacity.
• Better efficiency can be achieved by increasing the shoulder angle, shortening the case, and reducing the case taper (see internal ballistics).
• Greater consistency can be achieved by tuning the case capacity to a certain bullet diameter, weight, and velocity that give consistent results.
• Feeding problems can be fixed.

Some methods used to develop a wildcat are:

• Cold forming. The parent case is well lubricated and forced carefully into the reloading die for the wildcat caliber. This will swage the case into a new shape. This type of operation is used for reducing case dimensions, such as changing the neck diameter or pushing the shoulder back.
• Fire forming. This consists of taking the parent case, or a partially cold formed case, loading it with a light bullet and light load of powder, and firing it in the firearm it will be used in. Another technique uses a charge of fast-burning powder topped with a case full of Cream of Wheat and a wad, to form a special blank cartridge that will expand the case. This technique is used for increasing case dimensions, such as pushing the neck forward, increasing the neck angle, or straightening the case walls.^[5]
• Trimming to length. Generally, after either a cold forming or a fire forming operation, the mouth of the case will be longer than ideal, and the case will be trimmed back to the "trim to" length. Trimming is a normal reloading operation, as high-pressure cartridges will flow each time they are fired, and periodically need trimming to remove the brass that flows to the mouth.
• Changing the diameter of the case (to suit a new caliber). Called "necking up" or "necking down", this is the most common way of making a wildcat. The new caliber allows a different range of bullet weights, and can greatly increase the velocity or the power or the resistance to wind drift as compared to the parent cartridge.
• Necking back. This is a cold forming operation in which the neck is pushed back to reduce case capacity. This is often done when developing rounds for shorter barrels, such as turning a rifle cartridge into a handgun cartridge.
• Blowing out. This is a fire-forming operation that moves the shoulder forward to increase case capacity.
• Changing the shoulder angle. By making the shoulder closer to square, the resulting space is closer to the ideal spherical shape, resulting in a more efficient burn. If the shoulder is also to be moved back, this is a cold forming operation; if the shoulder is to stay or be moved forward, it is a hot forming operation.
• Reducing the case taper. This hot forming operation makes the cartridge more cylindrical, giving similar results to a shoulder angle change.
• Changing the rim. While this is a wildcatting operation, it is generally only done by commercial operations, due to the precision turning needed. Generally, this is a conversion from rimmed to a rimless cartridge, or from rimless to rebated, and is done to allow a larger parent case than the firearm action was designed for. The opposite operation, adding a rim to a case, is also generally only done by major manufacturers; examples are the .45 Auto Rim, a rimmed .45 ACP allowing ejection in .45 revolvers without the use of moon clips, and the .307 Winchester, a rimmed .308 Winchester, developed for use in lever-action rifles. A handloader can add a rim, by swaging a ring of metal onto a rimless case, then turning it down, but this is a very labor-intensive process and requires a special swaging die and precision metalworking lathe.^[6] It is far easier for most handloaders to simply start with a rimmed case, either of the desired diameter or reamed out as desired.
• Increasing the case length. This process (which allows the cartridge to contain more propellant and thus increases the potential energy of the bullet) was used to make the powerful .357 Magnum cartridge from the much weaker .38 Special: A .357 magnum bullet has more than 3 times more energy than a .38 special bullet of the same weight. Increasing the length of a bullet's case usually involves getting rid of the old case and making a completely new one from scratch, which all but limits the feasibility of this kind of modification for commercial manufacturers. It is possible to draw an existing case into a slightly longer form, thinning and stretching the existing case, but this is an operation requiring special equipment and expertise.^[6] It is far easier and more common to reduce, not extend the length of a case.

This section needs additional citations for verification. (January 2021)

In terms of sheer numbers of varieties, there are more wildcat cartridges than there are production cartridges. Most wildcats are custom made, therefore are not generally well-known. Some wildcat cartridges, however, are produced commercially in small quantities by small manufacturers. This is a list of some representative wildcats.

• .30 Herrett. Based on the .30-30 Winchester, necked back and with a sharper shoulder angle. Developed for use in pistols with barrels as short as 10 inches (25 cm), it develops the same power as a .30-30 with greater efficiency and less muzzle blast.^{[7][self-published source]}
• .357 Herrett. Like the earlier .30 Herrett, this cartridge is based on the .30-30 Winchester, shortened and necked up to .357 (9 mm). Designed for use in short barrels, the resulting cartridge is more efficient and more powerful than the .30-30. Often considered one of the best medium game hunting calibers available in the 10" (25 cm) barrelled Thompson Center Arms Contender pistol.^{[8][self-published source][9][self-published source]}
• .10 Eichelberger Long Rifle. This is one of a smaller number of wildcats based on rimfire cartridges. It is made by disassembling a .22 Long Rifle cartridge, and re-using the case. The .10 caliber (2.5 mm) is the smallest rifled barrel made. The tiny .10 caliber bullets produce almost no recoil and travel at very high velocities. While it can be used on small game at short ranges, this cartridge is more of a curiosity than practical hunting or target round.^[10]
• 5.7 MMJ, or 5.7mm Spitfire. A .30 Carbine case necked down to .223 caliber (5.56 mm), this cartridge was developed to convert military surplus M1 Carbines into short range varmint guns.^[8]
• 6mm PPC. Based on the .220 Russian, which is in turn based on the 7.62×39mm intermediate-power cartridge. The 6mm PPC was developed in 1975 specifically for benchrest shooting. While it is anything but common anywhere else, the 6mm PPC unseated the .222 Remington from its 20-year spot as the best benchrest cartridge available. Chambered only in single-shot rifles due to its short, fat case and sharp shoulder angle, the 6mm PPC is still going strong in benchrest after 30 years.
• .22 Eargesplitten Loudenboomer. This humorously named cartridge was developed by P. O. Ackley specifically to exceed 5,000 ft/s (1,500 m/s) muzzle velocity. Based on a .378 Weatherby Magnum case, the case is impractically overpowered for the bore diameter, and so the cartridge remains a curiosity.^[11][12]
• 7 mm TCU (also known as 7TCU). Based on the .223 Remington case, the 7 mm TCU is popular in single-shot handguns such as the Thompson Center Arms' Contender and G2 Contender. It is but one of a family of wildcat TCU cartridges.

• Small arms ammunition pressure testing
• Overpressure ammunition
• List of rebated rim cartridges
• C.I.P., a European standardization organization for firearm cartridges
• SAAMI, an American standardization organization for firearm cartridges
• NATO EPVAT testing
• DEVA, a German firearms test institute