American troops in Iraq and Afghanistan are discovering, through combat experience, what types of weapons work best at close range to take down the enemy. Same with SWAT teams and commandos all over the world. When conducting a raid, and you find yourself up close and personal with someone trying to kill you, there is a need for a heavy caliber pistol or a shotgun firing 00 shot or slugs. The premier pistol for ensuring you take down someone is still the .45 caliber (11.4mm) or .40 caliber (10mm, but only with a heavy bullet) pistol. These weapons are light and handy, compared to assault rifles or shotguns, and have a long history of quickly taking down an armed foe.
As the U.S. Army Special Forces discovered, if you are well trained and know what you are doing, you should carry a pistol, in addition to your rifle. But not the official issue 9mm pistol, but something with a bit more stopping power. The Special Forces prefer a new model .45 caliber pistol, although 10mm weapons are also popular. The reason for this is that you are most likely to be using the pistol indoors, where your target is going to be really close. You want to knock him down quickly, before he can get at you with a knife, or even his hands. Many troops are getting their own pistols, and most commanders have been lenient on this issue. The same applies to shotguns. Although the army and marines have bought a lot of them (the Benelli M4 Combat Shotgun is a particular favorite), there never seem to be enough of them for some units (that spend a lot of time raiding buildings in hostile neighborhoods.)
The troops also appreciate getting the most realistic urban combat training possible. This included the use of modified (to fire slower bullets that sting, but don't break the skin) pistols and rifles in "kill houses." Here, training can be carried out with live ammo. Kill houses are also equipped with vidcams, and the troops particularly like to watch the vids of their performances. Seeing your mistakes apparently makes it easier to correct them. For close combat, a weapon with stopping power is no good unless you know how to use it.
All of this stuff is old news to the Special Forces, which have been doing all of this for years. But the army and marine grunts are smarter, better trained and better led than at any time in the past. That's always been the description of the Special Forces, so it's not surprising that the better quality "regular infantry" are starting to adopt Special Forces techniques. SWAT teams everywhere pay attention to this military experience, as they often get involved in the same kind of close combat. SWAT operators are a large market, and driving the development of new equipment, which is often later picked up by military personnel.