What is a wildcat cartridge?

A wildcat cartridge is a custom made cartridge, usually made by modifying an existing commercial cartridge, in order to optimize cartridge performance (e.g. more power, more velocity, better accuracy etc.) Since these are custom made, they are more expensive than normal cartridges. Also, wildcat cartridges are usually used mainly by very serious shooters and hand-loading fans, there is only a limited market for them and larger cartridge manufacturers generally don't make them.

In terms of variety, there are more wildcat cartridge varieties than commercial production cartridges. However, many of these varieties are produced in very small quantities indeed.

In some cases though, some cartridges started out as custom-made wildcat cartridges, but gained enough popularity that they began to be commercialized (i.e. rifles chambered for them are now available commercially) and SAAMI standards were specified for them. Examples of such cartridges are the 6.8 mm. SPC, which was originally developed in collaboration with some members of US SOCOM. The 6.8 mm SPC is based on a .30 Remington cartridge, modified to .270 caliber and then further modified in length to fit in an magazine that can be fitted into the magazine wells of the M16 rifle. Therefore, any M16 or AR15 type rifle only needs replacing of the barrel, bolt and magazine to use this new cartridge. This cartridge is more lethal than the standard NATO 5.56x45 mm. cartridge fired by the M16 and while it is not officially adopted by the military, it has found use by special forces troops in Afghanistan and Iraq and is gaining popularity as a commercial civilian round. Another example is the well-known .357 magnum cartridge developed by Smith & Wesson, which was originally developed from a .38 Special cartridge. The .357 and .38 Special cartridge are both the same diameter externally and only differ slightly in length, because of safety reasons. Early versions of .357 magnum were actually identical dimensions to .38 special cartridges and the length was only altered so that people could not accidentally load the more powerful .357 magnum cartridges into a firearm not designed for the additional pressure. Another example is the 6 mm. PPC (Palmisano & Pindel Cartridge, named after its inventors, Lou Palmisano and Ferris Pindell). This cartridge started out as an improvement of the .220 Russian cartridge, which was itself based on the venerable 7.62x39 mm. cartridge used on the AK-47 and AKM assault rifles. The 6 mm. PPC case is made by forming the .220 Russian brass case into a new shape and is specially geared for single-shot bench rest shooting. It is one of the most accurate cartridges available up to 300 yard ranges and has been produced since 1975 and used in several competitions.

As mentioned earlier, wildcat cartridges are generally used by very serious shooters mainly and quite a few require barrel modifications also in order to use the modified cartridge. The modified barrels are usually supplied by custom barrel makers, who typically work out of small shops. The custom barrel makers generally also supply the buyer with reloading tools and dies, so that buyers can make their own cartridges. Some barrel makers also supply data about how different powder brands, powder quantities and bullet weights perform with their barrels. Therefore, most wildcat cartridges are developed, either by the custom barrel makers themselves, or by someone who is working in conjunction with a custom barrel maker.

.243 Winchester Ackley Improved wildcat cartridge on the left, compared to a normal .243 Winchester cartridge on the right.

Note the reduced case taper and sharper shoulder angle in the Ackley Improved version, which leads to more case capacity and therefore, more propellant.

Image copyright Arthurh at wikipedia and licensed under the Creative Commons Attribution-Share Alike 3.0 Unported License. 

There are many reasons why people develop wildcat cartridges, such as:

• Increasing the case capacity (as in the Ackley Improved cartridge pictured above) allows adding more propellant to the cartridge, which increases the velocity of the bullet and therefore, the energy transferred by the bullet.
• Reducing the bullet's caliber increases its velocity, thereby increasing its resistance to wind drift.
• Better consistency can be achieved by tuning a bullet's diameter, weight and velocity to a particular amount and type of propellant, which leads to greater accuracy.
• Feeding issues of certain types of ammunition can be fixed. For instance, it is not possible to reliably fire hollow-point bullets with .45 ACP pistol ammunition because of feeding issues from the magazine. Hence, the shape of the cylindrical cased .45 ACP cartridge was modified to a bottlenecked .45 cartridge to solve this issue.
• Some shooters like to use rifle ammunition with pistols, for greater accuracy. In this case, one starts out with a rifle cartridge and then reduces its case capacity so that it can be used with a pistol.

A wildcat manufacturer generally starts out by using a commercial cartridge case and changing its shape to new dimensions. Usually, this involves pushing the shoulder of the cartridge backwards or forwards as needed to modify the case capacity and also changing the diameter and length of the cartridge neck. This process can be done by either cold forming (i.e. the case is pushed into a die and pressure is applied to change the shape of the case) or fire forming (i.e. the case is placed in a chamber of a different dimension and loaded with a light gunpowder charge. Upon firing the charge, the case takes the shape of the new chamber). Sometimes a mixture of both methods is used to make the final case shape of a wildcat cartridge. Next, the manufacturer trims the case to the appropriate length, because cold forming or fire forming generally tends to increase the length of the case's mouth and the excess length needs to be trimmed. Then the diameter of the neck is changed as needed for the new bullet. The cartridge is then hand-loaded carefully and a bullet is crimped on.

In quite a few situations, firearms can be very easily modified to use the new wildcat cartridges. For example, the Ackley Improved cartridge shown above could easily be used by rechambering an existing firearm. Better still, a firearm that is chambered for the "Improved" cartridge can also fire standard factory loaded ammunition as well, which allows the owner to use less expensive and commonly available ammunition if there is a shortage of wildcat cartridges. Ackley Improved family of cartridges were developed by Patrick Otto Ackley, a prolific gunsmith and author, who produced many improved versions of commercial cartridges in several different calibers.