I've got a TA31F mounted on my 16" Carbine, and it works great. As others have said, the FSB is nothing to worry about. It all but disappears when looking through the scope, you are looking past it for your focus point, so it doesn't interfere with the sight picture. I commonly shoot mine from 10 yards to 300 yards, and I do pretty well with the Bindon Aiming Concept (both eyes open), so for me it works well even at close range. I won't say it's as fast as a red dot up close, but for me it works fine, as I'm not clearing rooms for a living. I've considered mounting a red dot on my ACOG, but I prefer not to increase the height of the weapon, however, I'm still considering it at some point in the future. I went with the 31F over the standard TA31DOC or TA31TRD (red dot already mounted) as when looking through several different ones, I liked the look of the Chevron reticle over the standard crosshairs found on the red dot mounted ACOG units, as I find the multiple crosshairs too "busy" for my vision. Also, as Don't Tread stated, you won't want the TA55 on that gun, it's a much larger scope (in physical size) than the 4x units, there was a pic on here somewhere of one mounted on an AR10 308, with a 20" barrel, and it still looked rather large sitting on top of that rifle. The 4x will be perfect for what you're wanting to use it for.