There are some powders on the market now that allow those muzzle velocities. Alliant says 2,725 from a 24" barrel with Power Pro 2000-MR and 2,782 using AR Comp. Assuming 25fps per inch then that would be about 2,625 or 2,682 from a 20" barrel (would really need to confirm over a chronograph).
Hogdon lists 2,828 fps with CFE 223, and assuming that we lose 150 fps from a 26" barrel (my assumption as to how that velocity is so high) to a 20" barrel that is still a respectable 2,678 fps.
Still, those are max charge loads and not a significant performance increase over a standard service load of M852.
Machine guns are awesome until you have to carry one.