Here was a good idea that was for whatever reason was not adopted.
The problem is this: Pistols are hard to shoot very well. Especially if you don't practice much or are under stress. So a pistol as a personal defense weapon is not really the best idea. They have very little accuracy beyond 50 feet or so, almost no ability to breach body armor, and have relatively small magazines, as a rule. The only thing going for them is they are small and light. Many Soldiers, Sailors, Airmen, and Marines such as pilots, vehicle crews, mechanics, medics, artillerymen and so forth need a weapon, but are not really expected to use it in less there is a real emergency. The M16 series, relatively long and heavy and maintenance intensive, is not the best choice. What is needed is a weapon designed (or modified) for the PDW role, it needs to be light and small enough to not be in the way during the soldiers' main tasks, yet accurate, reliable and powerful enough to neutralize the enemy. It would also be nice to be in a caliber already in use by the force in question, so we don't make the poor logistics people have a fit.
FN and H&K release select fire Personal Defense Weapons (PDW) designed to replace the common 9mm sidearms issued to many support troops
There are some who think that the new PDW cartridges (5.7x28mm and 4.6x30mm) are not powerful enough. The argument is that the terminal effectiveness is compromised by the fact that these light bullets do not "dump" enough energy into the target. Weather that is true, I don't know. But it is true to say that both cartridges were designed to be high velocity, armor piercing bullets. In the past many have said that small diameter, high velocity bullets are less effective than a larger, heavier bullet.
H&K Mp7, 4.6x30mm
Enter the Crist PDW concept. It replaces the barrel and slide of the Beretta 92 series, which is the official sidearm of the US Armed Forces. It gives the user a longer barrel which gives higher velocity and increased accuracy. Along with a collapsible butt stock and space to use a true two handed grip, the effective accuracy of the Crist PDW would be much higher than the standard M9. And that doesn't even bring into the equation the provision to mount a red dot sight on the rail. On my recent vacation, I fired a 9mm Beretta Storm carbine and its accuracy was enough to hit small targets at 100 yards, more than enough for the PDW role. Another strength of the Crist system is that it would not necessitate a change of ammunition. Anyone who follows US military technology would be forced to assume that the Pentagon plans on using the 5.56mm and 9mm for the next 300 years or so. The Crist is also small enough to be worn on a leg holster. The big upside for the Department of Defense is that they can modify weapons already in stock, use the same ammo and magazines, and still get a better weapon that is needed.
This is another prospective PDW, the VBR-B. It is from Belgium and was designed by Rik Van Bruaene. It is a select-fire weapon with a collapsible stock and uses common and reliable Glock magazines. It is available in 9mm NATO and a 7.92x24mm VBR cartridge. The 7.92 VBR was designed to function as a armor-piercing round that has superior terminal effectiveness to the 5.7 and 4.6. There are two bullets lengths, one designed for 9mm platforms and the other for the .45ACP. The standard FMJ cartridge is a shortened .30 Carbine case and uses .32 ACP bullets. Seems pretty neat to me. It is still in prototype at this time, but I think it has promise.