From my buddy at the Firearm Blog; apparently the FBI has decided that hey, pistol caliber carbines are kind of a neat idea. According to the procurement document, the FBI wants these carbines for training and as option to the 5.56 AR15 rifles that they currently issue. The caliber makes sense as well, as the FBI’s standard issue firearms for Special Agents is usually a Glock 22 in .40 S&W. The FBI document actually mentions “easily sourced ammo” as a reason for going with the .40 S&W AR.
The actual rifle selected is Rock River Arms LAR-40. I personally haven’t played with this gun, but I’ve shot the RRA 9mm carbine, and it was absolutely a fine gun. I also think that .40 makes a lot of sense as a carbine/sub-gun round as the high pressures that the round operate at and somewhat snappy recoil out of certain pistol platforms is easily mitigated by the carbine. You could do worse than to imitate the Feds and get one of these for your HD gun.