No U.S. Law Enforcement has widely adopted the AK as a duty weapon, the CLEO (Chief Law Enforcement Officer) of some of the smaller agencies have allowed their officers to carry a personally owned AK while on duty, in the trunk. These are usually set up in tactical configuration and are semi autos, 922r compliant, and used primarily in rural areas.
I have built (Overland Industries) several of these configurations with top quality new parts, additional modification for improved reliability and accuracy, faster mag changes, quad rail for tailoring the configuration easily, ergonomic modifications, and are in 7.62 x 39 caliber, these have been carried for years and none have ever been fired on duty.
Since these rifles are owned by the officers they still use them for target practice and sporting use. Many of these officers have registered SBR's in their collections however, they never use or take them while on duty. These rifles are not built as Sporter rifles as the officer may have their life on the line and a failure is not an option.
Most SWAT teams will use AR's (and an occasional FAL, M-40, or another rifle that has been approved for their use) some in full auto however, most in semi auto with the Colt 6920 being the most socially accepted choice. The Smith and Wesson M&P is also starting to gain more acceptance along with other M&P models being used. The reason for this choice is most LEO's have a U.S. military background and are familiar with the AR and therefore the AR makes for an easy choice. So this choice is highly political.
The main reason for the decision to not allow field officers to carry a high powered rifles (other than the old shotgun) is the legal liability (plus most departments do not have the budget for these additional purchases). Even when the officers are willing to pay for their own firearm, many CLEO's are not willing to accept the liability or present the offer to the City Manager or the City Council for their approval (they usually turn the request down). One collateral injury would likely end the careers of the CLEO, the City Manager, and the City Counsel/Mayor (their logic, why chance it?) from a lawsuit.
In the European theater, Krinkov's and UZI are a very common sight used by many of the Law Enforcement Agencies (not just Interpol) as a everyday weapon and no one even looks twice.