Why do American troops prefer the century old .45 caliber pistol to lighter 9mm models. It's all about "stopping power." As far back as 1900, during the fighting against Moslem rebels in the Philippines, there had been complaints of enemy fighters getting shot and continuing to attack. From this experience, the then standard army pistol, a .38 caliber (actually .357, or 9.07mm) weapon, was replaced by a .45 caliber (11.4mm) one. In the 1980s, the .45 was in turn replaced by a 9mm pistol. There were a lot of complaints about that, but 90 years of experience demonstrated that you should not depend on a pistol in the heat of combat. But that was largely European experience, in major wars. In these conditions, pistols were rarely used in desperate battles. The fighting in Iraq reminded everyone that, especially in urban combat, a pistol was still an essential weapon. Going into buildings, troops would often prefer to have one or two guys holding pistols, as these could get into action faster if you were in cramped and crowded conditions. At close range, you didn't want someone with a gun, or a knife, to require a second shot. And at ranges measured in a few feet, you couldn't miss. If the enemy was amped up, you wanted to take him down with one shot, because there might not be time for a second. Many police SWAT teams have stayed with the .45 for the same reason.
The M1911 was better at stopping people, and that was mainly due to the size of the bullet. Technically, the "hitting power" of a bullet is determined by the bullet weight and velocity, and is measured in joules. The widely used 9mm Parabellum generated 583 joules, the Russian 7.62mm Tokarev (mainly used to execute cowardly soldiers, POWs or uncooperative civilians) produced 499 joules, while the .45 (11.4mm) only came up with 450 joules. But there's a major problem in just using joules, and that is how much of that energy is actually applied to the person being hit. A smaller, faster bullet has a tendency to just go through someone. This does damage, often fatal damage, but if often does not slow down a highly energized soldier. A larger bullet, especially a blunt one, will be more effective at "stopping" someone. Thus the popularity of the .45 caliber pistol round. Although it has less energy than the 9mm round (450 joules compared to 583), those who have used both insist that the .45 is far more effective than the smaller and faster 9mm. Part of this has to do with the fact that the .45 (11.4mm) bullet hits with a 60 percent larger (as seen head on) area, thus it applied more of that energy to the target. This explains the greater likelihood of the .45 caliber bullet "knocking down" whoever it hits. The same physics applies to rifle bullets (although they tend to have pointy tips, unlike the blunter ones for pistol pullets.) A 7.62mm bullet is 88 percent larger (head on) than a 5.56mm one.
Even before the Department of Defense decided to switch back to the .45, SOCOM (Special Operations Command) and the U.S. Marine Corps went and got .45 caliber pistols for use as an "alternative" to the standard 9mm M9. SOCOM was never happy with the 9mm's pistol's stopping power, even in the very limited scenarios, such as terrorist hostage rescue, where they can legally use 9mm hollow-point ammunition for increased effectiveness. SOCOM went out and developed the HK Mark23 Mod 0 SOCOM "offensive" handgun weapons system. This weapon, based on a popular H&K design, is 1.53 inches wide, 5.9 inches high and 9.65 inches long. It weighs 2.42 pounds empty and uses ten or twelve round magazines. The original M1911 is 8.25 inches long, 5.25 inches high, 1.5 inches wide and weighs 2.44 pounds empty (add .4 pounds for a loaded, seven round, magazine). Some 2.7 million M1911s have been manufactured so far, 1.9 million of them during World War II. Some 650,000 of the new U.S. .45 caliber pistols are expected to be manufactured initially.
The U.S. Marine Corps have been using M1911s rebuilt from the many old ones turned in when everyone switched to the M9. But this supply is running out, and the marines have been eager to see the 9mm M9 pistol replaced with a new .45 caliber model. Some marines (and other troops) buy these newer .45 caliber weapons with their own money. Most American combat units tolerate troops bringing in some additional weapons, especially pistols. Some troops have been buying 10mm pistols, seeing this as a nice cross between the lighter weight of the M9 (2.55 pounds versus three for the .45) and the greater stopping power of the 11.4mm M1911 bullet. But there are new .45 models that weigh as much as the M9, carry more bullets (10) and are easier to repair than the M1911.
The SOCOM Mk 23 may not be a prime candidate for the new standard pistol. That's because the Mk 23 is a large weapon. A new "standard .45" will be used by a wide variety of troops, including women (who have smaller hands.) It is possible to make smaller .45s. One of the smallest currently available is the Glock Model 37. This .45 caliber pistol is 7.32 inches long, 5.51 inches high and 1.18 inches wide. It's 1.63 pounds empty, and 2.22 pounds with a ten round magazine. Glock began making .45 caliber pistols in the early 1990s, and has steadily improved that design. There are smaller .45s than these Glocks, but none that are as sturdy and reliable. So it is possible to get a smaller .45 design that will be as robust as the original M1911.