Back in August (2009) the Department of Justice published their intention to purchase AR-15 carbines chambered in .40 S&W from Rock River Arms.

Civilian model of the LAR-40. FBI will probably use a short barreled version.

Their reasons for choosing the RRA LAR-40 carbine is that, because they already use AR-15s, training and maintenance will be minimized. From the solicitation notice …

BASED ON THE MARKET RESEARCH AND FBI REQUIRMENTS, , USE OF A COLT PATTERN .40 S&W CALIBER CARBINE, WILL RESOLVE THE MAINTENANCE ISSUE, ENSURING A RELIABLE SOURCE OF PISTOL CALIBER CARBINES. FURTHER, THE ROCK RIVER ARMS LAR40 PISTOL CALIBER CARBINE WILL ALLOW THE FBI TO MAXIMIZE TRAINING, SINCE THE OPERATIN SYSTEM AND CONTROLS ARE VIRTUALLY IDENTICAL TO THE COLT PATTERN 5.56 X 45 ADDITION, BY USE OF A COMMON WEAPON PLATFORM, THERE IS A REASONABLE AMOUNT OF PARTS INTERCHANGEABILITY BETWEEN EXISTIN FBI 5.56 X 45 MM LAR15 ROCK RIVER ARMS CARBINES AND THE LAR40, FURTHER ENHANCING THE COST EFFECTIVENESS OF THIS ACQUISITION. THE FBI REQUIRES AN INTEGRAL MAGAZINE WELL (AS OPPOSED TO A PINNED MAGAZINE WELL ADAPTOR) AND A CALIBER DEDICATED MAGAZINE, DEVELOPED SPECIFICALLY FOR THIS WEAPON SYSTEM AND CALIBER, WHICH ENSURES THE LEVEL OF RELIABILITY REQUIRED BY FBI SPECIAL AGENTS DURING HIGH RISK SITUATIONS. THE FBI REQUIRES THIS CARBINE TO BE MANUFACTURED IN .40 S&W CALIBER, WHICH WILL PROVIDE GREATER OPERATIONAL EFFECIENTCY, SINCE BOTH THE ISSUED SERVICE PISTOL AND THE PISTOL CALIBERCOLT PATTERN CARBINE WILL BE CHAMBERED FOR THE SAME AMMUNITION (I.E., .40 S&W CALIBER). ROCK RIVER ARMS IS THE ONLY MANUFACTURER KNOWN TO MEET THESE CRITICAL REQUIREMENTS.

You have got to hand it to the FBI, first 20mm rifles and now AR-15 carbines chambered in .40 S&W, they do think outside the box.

[ Many thanks to jdun1911 for emailing me the link. ]