.30 Carbine, 5.45mm x 39mm
The .30 carbine case has a number of attributes that makes it an ideal cartridge case of choice. Overall, it's length is only approximately 1.6 inches with a bullet loaded, making it roughly comparible to the .357 magnum, .44 magnum, and 5.7mm x 28mm and 4.6mm x 30mm series of bullets. More or less the longest round a weapon can conceivably possess without being overly large, it is at the maximum size one can comfortably fit in their hand, such as within a pistol or handguard of a submachine (like an uzi or MAC-10). Some popular series of pistols, such as the
AutoMag, successfully employed the .30 carbine in this way. Despite this, the energy is much higher. Compared to the 5.7mm, 4.6mm, and even the .357 magnum, it possess much more energy. With 500, 400, and 1,000 joules respectively within each cartridge, the .30 carbine has 1,300 joules. While this is out of a longer barrel, it quite easily can maintain well over 1,000 joules out of a pistol sized weapon. On top of this, the weapon had just 38,500 PSI of pressure, compared to 41,000 PSI for the .357 magnum and .44 magnum, and 50,000 and 58,000 PSI for the 5.7mm and 4.6mm, respectively. The lower pressures allow far weaker and softer gun parts to be used in the creation of the firearm, such as those found commonly in pistols and other similar handguns. The base diameter of the cartridge is just 9.2mm, compared to 9.96mm for the 9mm, and the round is only 12.3 grams loaded, nearly identical the 9mm parabellum, despite possessing well over double the energy and using a bullet of the same size. Despite this, the round has much greater recoil, given it's greater power and size, making it harder to use.
By comparison, the 5.45mm has approximately the same level of energy from the same length barrel, or 1,400 joules. The round is only 3.33 to 3.9 grams, compared to 7.5 grams, and is somewhat more aerodynamic, giving it a longer range. More importantly, the round is capable of piercing armor, equal to most rifles such as the .223 or 5.56mm, and thus can penetrate common barriers or light body armor, such as a helmet. The round however uses the Ak-47 case. Fully loaded, it is 10.75 grams, 2.25 inches long, and has 55,000 PSI. It is much too large to be held in a single hand, and has higher pressures and weight. The .30 carbine round simply has better ballistic. If a .30 carbine cartridge was necked down to 5.45mm, it could maintain the same level of ballistics but instead be capable of being fired out of pistols or submachine guns. It would also have less recoil. In fact, it would be nearly equal to the 5.7mm and 4.6mm in terms of length, with a slightly wider bullet, making the magazine capacity slightly reduced, but still comparable, and greater than a 9mm.
By comparison, out of a 2.5 pound pistol, a .45 ACP and 9mm round have approximately 10.56 and 5.22 foot-pounds of
recoil force in a 2.5 pound pistol. The 5.45mm has 6.40, which while still higher than the 9mm, is still much lower than the .45. Roughly equal to a .40 S&W, it's recoil would be more than easily manageable in a medium to large sized pistol, and it's overall length and size would not be too large to fit in such handguns. It's lower pressure than the 5.7mm or 4.6mm would allow it to operate without much issue, despite possessing nearly double the energy from a handgun, and triple that from a rifle. In addition, the weight of the cartridge would be below that of the 5.45mm x 39mm, at 9.4 grams, and even less than the 9mm x 19mm parabellum, the standard NATO handgun cartridge.
If the chamber pressures were maxed out, to 50,000 PSI and slightly more powerful, modern gunpowder was used (past WWII era gunpowder from the 1940's ), then the weapon could be slightly more powerful. Generating 1,600 joules from a 16 inch barrel, the weapon could be comparable to rifle rounds, despite still being the size of a pistol round. While nowhere near as powerful as many more powerful rounds, it's role as a light-weight, armor piercing round would put it far past many other weapons, such as the 4.6mm and 5.7mm, with a longer range, heavier bullet, more energy, and even better armor penetration.. Faster burning propellant that burns more quickly in the barrel would allow it's use with shorter barrels to be more effective, such as with modified
5.56mm rounds. Overall, it's ability to be used both as a light submachine gun and heavy pistol round would be greatly improved over both of the original cartridges. Compared to a 9mm, it would be approximately 32% lighter, and possess approximately 10% greater magazine capacity. While the recoil could be slightly greater, this would be easily manageable and, easily reduced with a recoil buffer or muzzle break.
