I have both:
the 858-4V, restricted, chrome lined 390mm (15.5") barrel, folding stock
the 858-2, non- restricted, non-chrome lined 477mm (18.4") barrel, fxked stock.
The factory target provided with the longer one is significantly better grouped (2 inches instead of 4). I don't believe this to be statistically different though, comparing just two string of six rounds. I have only tried the shorter one so far, with the surplus S&B ammo. This ammo, by the way is very constant. I clocked it at 2341 fps with standard deviation of only 14 fps. Got 2" groups at 50m, consistent with factory target.
I bought the muzzle brake with it, which I believe is a must. I also have the scope mount, so part of my future testing is going to remove some of the shooter's (me) inadequacies out of the equation. More results later.
This is a fantastic rifle, nothing can go wrong with it. I bought from a CNG machinist a bolt carrier with the handle on the left side, like on my MP-44. The looks with the folding stock is just fabulous. The longer non-restricted looks just as great.
My only advice, hurry, get one before the supply dries up or they get banned. If they come up with a chrome barrel on the longer one (Marstar is trying to get them), I will probably upgrade my -2 version.
Cheers.