I'd have to agree with Zach. Beyond the historical aspect, a 20" barrel isn't going to do much. If you're wanting to get the most out of 5.56, a 24" barrel would serve you better as that's going to give you about all you're going to get.
A 16" barrel is much handier, and a 14.5" barrel with either a pinned/welded flash hider, or in integrated flash hider even more so. The difference between 16" and 20" is maybe 100-200 fps depending on the load.
If you're not doing anything but shooting off of a bench with it, then the barrel length and the extra weight and bulk associated with it doesn't matter. If you plan on using it for home defense or plan on lugging it around the woods on long hunts, then the shorter barrel lengths might be a better option for you.
Even a mediocre 16-inch barrel will do "minute of looter/arsonist/vicious murderer" out well past 150M.
(I'm looking for another Model 7615 Police Carbine by Remington to replace the one that my lady "adopted". - It did FINE from a prone, kneeling or sitting position out to over 200M,with a variety of GI/commercial/reloads & it emptied 20-round M16 mags as QUICKLY as I could fire & reacquire the target.)