Really nice to have the fsb with the free float - Aero Precision M4E1 10.5in 5.56 Barrel w/ FSB Complete Upper Receiver with Flash Hider

I think this is the first upper Aero has made that has incorporated the FSB but also a M-lok and free-float rail rather then have the drop-in hand guards. Just looks better and feels better, a little more secure for attachments. I really like the look of the FSB uppers. Reminds me a lot of the different BCM options.