The AR-15 is easily the most popular Rifle in the US, and its standard caliber, the 5.56 Nato, or .223 Remington caliber is also the most popular rifle caliber in the US. Therefore, 5.56 Casings are quite abundant. Ammo Manufacturers have the gear and machinery to make these cases in large amounts, and most shooters who reload their casings also probably have a lot of 5.56 Casings laying around.\nTherefore, Many Ammo manufacturers like to use the standard 5.56\/.223 platform to make new calibers. Since they use the same casing as a 5.56 round, fit in standard AR mags, and in some cases can even be shot using the same bolt as a standard AR-15.\nSo, today we are going to talk about some of the most popular Wildcat Calibers that you can make using the standard 5.56\/.223 platform\n.300 Blackout:\nThe .300 BLK is the most popular WIldcard caliber based on the 5.56 platform. It was developed by Advanced Armament Corporation for US Special Forces, as an intermediate, high-power cartridge for close-range combat. .300 blackout is made by stretching out a standard 5.56 casing and shortening it. It is loaded with fast-burning powders and uses a 30-caliber projectile. The .300 Blackout is an excellent home defense caliber, and it can be used for hunting as well.\n7.62×40 mm:\nThere are multiple 7.62x40mm cartridges made on the 5.56\/.223 platform, however, the most common ones are 7.62x40mm WT and 7.62x40mm Kurt.\nThe 7.62x40mm WT (Wilson Tactical) was made in 2011 by a company named Wilson Combat. Their goal was to make a cartridge that used a .30 Caliber projectile for hunting that had low recoil and could work with the M4\/Ar-15 platform with as few part changes as possible. Therefore, it used the standard 5.56\/.223 casing.\nThe 7.62×40mm WT delivers more energy on target than a standard 7.62×39mm round, and is more close to 6.8 SPC and .30-30 Winchester in terms of ballistics and energy. It is an excellent caliber for hunting deer and feral hogs, especially for hunters who like to use AR-15s, but can’t legally use the 5.56 cartridge.\nThe 7.62x40mm Kurt was also developed for similar reasons by Kurt Buchert of Lake Charles, Louisiana, however. It isn’t as popular as the 7.62x40mm WT\n6x45mm\nThe 6x45mm caliber is one of the easiest calibers to reload using the 5.56\/.223 platform. It simply uses a necked-up version of the .223 Remington casing, without any further modifications or improvements.\nHowever, with a larger projectile, the 6x45mm cartridge is capable of being loaded with heavier bullets with better ballistic coefficient ratings than a standard 5.56 or .223 round. Moreover, most states in the US require you to use a minimum of .24 caliber projectile for hunting, and since you can’t use a standard .223 caliber, the 6×45 is a better option with its higher stopping power and muzzle velocity.\nTCU Family of Calibers:\nThe TCU family of calibers is also based on the 5.56\/.223 platform. These calibers were developed for the Thompson Center Arms Contender single-shot pistol by a person named Wes Ugalde of Fallon, Nevada. They used a standard .223 Casing and necked it up to accept larger calibers.\nThe most common Thompson Center Uvalde calibers are based on the 5.56\/.223 platforms are:\n\n6 mm TCU\n25 Ugalde or .25 TCU\n5 mm TCU or 6.5×45 mm\n7 mm TCU\n.30 TCU\n\nHonorable mentions:\nApart from the calibers mentioned above, there are several other calibers that are based on the 5.56\/.223 platform. However, they aren’t as popular as the ones mentioned. Here is a list of some other calibers based on the 5.56\/.223 platform.\n\n.25-45 Sharps\n6mm Mongoose\n.357 MAX AR\n338 Thumper\n\nFinal thoughts:\nDue to the wide availability of military surplus 5.56\/.223 casings, many ammunition manufacturers have based their calibers on this parent casing. So, if you have a lot of .223 casings laying around, know that they are very versatile, and can be loaded into a wide range of calibers.