Arms Unlimited has a small quantity (about 49 as of 27 MAY 2022) of the elusive Colt LE6920-FBP2 carbines. These carbines are equipped per FBI contract specification with a Geissele rail and Magpul Pro back-up iron sights just to name a couple of the features. There have only been two or three hundred (rough guestimate based on observed quantities released) of these made available to the market to date.
Here are some photos of my Colt LE6920-FBP2 for anyone not familiar with the carbine.
The FBP2 carbine is one of my top three favorite rifle/carbines along with the 6920-EPR and the AR-15A4.
You can get yours here at this link: https://www.armsunlimited.com/Colt-M4-Carbine-Federal-Patrol-Rifle-p/le6920-fbp2.htm?CartID=1