One of the first pistol caliber subguns favored by law enforcement was the 9mm Walther MPK, which ceased production in 1985. When the Walther was no longer available, armorers outfitted their teams with a wide variety of select-fire and full-auto weapons, including the 9mm Smith & Wesson M76 and the 9mm UZI. Today the most common subguns used by American law enforcement are the 9mm Heckler & Koch MP5 in 9mm, .40 S&W, or .45 ACP, and the exotic FNH P90 in 5.7x28mm, which is a favorite of the Secret Service and the FBI.
After the infamous 1997 North Hollywood Shootout in Los Angeles where the bad guys were equipped with armor that stopped pistol rounds, law enforcement agencies began adopting M16s, AR-15s, and M4s as "patrol rifles." This meant that an increasing number of regular patrol personnel began carrying a semi-automatic 5.56mm M4.
But many tactical officers stuck by the pistol-caliber subgun. They liked the quantity of fire that the subguns could concentrate on target, and they liked the compact form of the guns.
This started to change in the last five years, as more and more tactical operators began to favor the M4 for its rifle-caliber power and combat-proven performance in Iraq and Afghanistan. However, many officers still wanted compact entry weapons, so they opted for short-barreled M4 variants.
Even though I am a big fan of using semi-automatic and select-fire rifles and carbines in 7.62mm NATO, for certain applications short-barreled M4s chambered for 5.56mm ammo are the best option. This is especially the case when a tactical team or any law enforcement officer is engaging an armed subject inside a dwelling.